feat: subnet printer discovery (Feature 3)

- New GET /api/system/printers/scan SSE endpoint: parallel async TCP
  scan of a /24 subnet, streams found/progress/done events
- New GET /api/system/printers/scan-hints: returns auto-detected local
  subnets from socket.getaddrinfo, excludes loopback/docker ranges
- FindPrintersModal in Settings > Print: shows subnet+port inputs,
  live progress bar, found printers list; selecting a result pre-fills
  the new printer form (user still names it before saving)
- "Εύρεση εκτυπωτών" button added next to "+ Νέος εκτυπωτής"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/routers/system.py

@@ -1,5 +1,10 @@
+import asyncio
+import ipaddress
+import json
+import socket
 import time
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi.responses import StreamingResponse
 from sqlalchemy.orm import Session
 from typing import List
 
@@ -164,6 +169,103 @@ def delete_printer(printer_id: int, db: Session = Depends(get_db), user: User =
     return {"ok": True}
 
 
+def _detect_local_subnets() -> list[str]:
+    """Return plausible /24 subnets based on local interface IPs, excluding loopback/docker."""
+    subnets = []
+    try:
+        hostname = socket.gethostname()
+        for info in socket.getaddrinfo(hostname, None):
+            ip_str = info[4][0]
+            try:
+                addr = ipaddress.IPv4Address(ip_str)
+                if addr.is_loopback or addr.is_link_local:
+                    continue
+                # Exclude docker bridge ranges (172.16-31.x.x)
+                if addr.packed[0] == 172 and 16 <= addr.packed[1] <= 31:
+                    continue
+                net = str(ipaddress.IPv4Network(f"{ip_str}/24", strict=False))
+                if net not in subnets:
+                    subnets.append(net)
+            except (ValueError, OSError):
+                continue
+    except OSError:
+        pass
+    return subnets
+
+
+async def _check_port(ip: str, port: int, timeout: float) -> bool:
+    try:
+        _, writer = await asyncio.wait_for(
+            asyncio.open_connection(ip, port), timeout=timeout
+        )
+        writer.close()
+        try:
+            await writer.wait_closed()
+        except Exception:
+            pass
+        return True
+    except Exception:
+        return False
+
+
+async def _scan_subnet_sse(subnet: str, port: int, token: str):
+    """Async generator: yields SSE lines as printers are found."""
+    try:
+        network = ipaddress.IPv4Network(subnet, strict=False)
+    except ValueError as e:
+        yield f"data: {json.dumps({'error': str(e)})}\n\n"
+        return
+
+    hosts = list(network.hosts())
+    semaphore = asyncio.Semaphore(50)  # max 50 concurrent connects
+
+    async def check(ip_str: str):
+        async with semaphore:
+            return ip_str, await _check_port(ip_str, port, timeout=0.4)
+
+    tasks = [asyncio.create_task(check(str(h))) for h in hosts]
+    yield f"data: {json.dumps({'type': 'start', 'total': len(tasks), 'subnet': subnet, 'port': port})}\n\n"
+
+    done_count = 0
+    for coro in asyncio.as_completed(tasks):
+        ip_str, reachable = await coro
+        done_count += 1
+        if reachable:
+            yield f"data: {json.dumps({'type': 'found', 'ip': ip_str, 'port': port})}\n\n"
+        if done_count % 20 == 0 or done_count == len(tasks):
+            yield f"data: {json.dumps({'type': 'progress', 'done': done_count, 'total': len(tasks)})}\n\n"
+
+    yield f"data: {json.dumps({'type': 'done'})}\n\n"
+
+
+@router.get("/printers/scan")
+async def scan_printers(
+    subnet: str = Query(default=""),
+    port: int = Query(default=9100),
+    user: User = Depends(require_manager),
+):
+    """SSE endpoint: scan a subnet for devices responding on the given port."""
+    # Fall back to auto-detected subnet if none provided
+    if not subnet:
+        detected = _detect_local_subnets()
+        subnet = detected[0] if detected else "192.168.1.0/24"
+
+    return StreamingResponse(
+        _scan_subnet_sse(subnet, port, ""),
+        media_type="text/event-stream",
+        headers={
+            "Cache-Control": "no-cache",
+            "X-Accel-Buffering": "no",
+        },
+    )
+
+
+@router.get("/printers/scan-hints")
+def scan_hints(user: User = Depends(require_manager)):
+    """Return auto-detected local subnets to suggest in the scan UI."""
+    return {"subnets": _detect_local_subnets()}
+
+
 @router.get("/stats")
 def system_stats(db: Session = Depends(get_db), user: User = Depends(get_current_user)):
     return {

manager_dashboard/src/pages/Settings/tabs/PrintFontsTab.jsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react'
+import React, { useState, useEffect, useRef, useCallback } from 'react'
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import toast from 'react-hot-toast'
 import client from '../../../api/client'
@@ -774,10 +774,174 @@ function PrinterRow({ printer, onEdit, onDelete, onTest, onToggle, testPending }
   )
 }
 
+function FindPrintersModal({ onClose, onSelect }) {
+  const [subnet, setSubnet] = useState('')
+  const [port, setPort] = useState(9100)
+  const [scanning, setScanning] = useState(false)
+  const [found, setFound] = useState([])
+  const [progress, setProgress] = useState(null) // { done, total }
+  const [error, setError] = useState(null)
+  const esRef = useRef(null)
+
+  // Fetch auto-detected subnets on mount
+  useEffect(() => {
+    client.get('/api/system/printers/scan-hints')
+      .then(r => { if (r.data.subnets?.length) setSubnet(r.data.subnets[0]) })
+      .catch(() => {})
+  }, [])
+
+  const stopScan = useCallback(() => {
+    if (esRef.current) { esRef.current.close(); esRef.current = null }
+    setScanning(false)
+  }, [])
+
+  useEffect(() => () => stopScan(), [stopScan])
+
+  function startScan() {
+    stopScan()
+    setFound([])
+    setProgress(null)
+    setError(null)
+    setScanning(true)
+
+    const token = localStorage.getItem('access_token') || ''
+    const params = new URLSearchParams({ subnet, port })
+    // Use fetch with ReadableStream to consume SSE (EventSource doesn't support auth headers)
+    fetch(`/api/system/printers/scan?${params}`, {
+      headers: { Authorization: `Bearer ${token}` },
+    }).then(async res => {
+      if (!res.ok) { setError('Σφάλμα σύνδεσης'); setScanning(false); return }
+      const reader = res.body.getReader()
+      const decoder = new TextDecoder()
+      let buf = ''
+      while (true) {
+        const { done, value } = await reader.read()
+        if (done) break
+        buf += decoder.decode(value, { stream: true })
+        const lines = buf.split('\n')
+        buf = lines.pop()
+        for (const line of lines) {
+          if (!line.startsWith('data: ')) continue
+          try {
+            const msg = JSON.parse(line.slice(6))
+            if (msg.type === 'found') setFound(prev => [...prev, { ip: msg.ip, port: msg.port }])
+            else if (msg.type === 'progress') setProgress({ done: msg.done, total: msg.total })
+            else if (msg.type === 'done') setScanning(false)
+            else if (msg.error) { setError(msg.error); setScanning(false) }
+          } catch {}
+        }
+      }
+      setScanning(false)
+    }).catch(e => { setError(String(e)); setScanning(false) })
+  }
+
+  const pct = progress ? Math.round((progress.done / progress.total) * 100) : 0
+
+  return (
+    <div style={{
+      position: 'fixed', inset: 0, background: 'rgba(0,0,0,0.45)', zIndex: 1000,
+      display: 'flex', alignItems: 'center', justifyContent: 'center', padding: 20,
+    }} onClick={e => { if (e.target === e.currentTarget) onClose() }}>
+      <div style={{
+        background: 'white', borderRadius: 14, width: '100%', maxWidth: 520,
+        boxShadow: '0 20px 60px rgba(0,0,0,0.2)', overflow: 'hidden',
+      }}>
+        <div style={{ padding: '20px 24px 16px', borderBottom: '1px solid #f0f1f3' }}>
+          <h2 style={{ fontSize: 17, fontWeight: 700, color: '#111315', margin: 0 }}>Εύρεση εκτυπωτών</h2>
+          <p style={{ fontSize: 12, color: '#9ca3af', margin: '4px 0 0' }}>
+            Σκανάρει το δίκτυο για συσκευές που ακούνε στη θύρα εκτύπωσης.
+          </p>
+        </div>
+
+        <div style={{ padding: '16px 24px', display: 'flex', gap: 10, alignItems: 'flex-end', flexWrap: 'wrap' }}>
+          <div style={{ flex: '1 1 220px', display: 'flex', flexDirection: 'column', gap: 4 }}>
+            <label style={{ fontSize: 11, fontWeight: 600, color: '#6b7280' }}>
+              ΥΠΟΔΙΚΤΥΟ <span style={{ fontWeight: 400, color: '#b8bdc4' }}>(π.χ. 192.168.1.0/24)</span>
+            </label>
+            <input
+              value={subnet}
+              onChange={e => setSubnet(e.target.value)}
+              placeholder="192.168.1.0/24"
+              style={inputStyle}
+              disabled={scanning}
+            />
+          </div>
+          <div style={{ flex: '0 0 90px', display: 'flex', flexDirection: 'column', gap: 4 }}>
+            <label style={{ fontSize: 11, fontWeight: 600, color: '#6b7280' }}>PORT</label>
+            <input
+              value={port}
+              onChange={e => setPort(parseInt(e.target.value) || 9100)}
+              type="number"
+              style={inputStyle}
+              disabled={scanning}
+            />
+          </div>
+          <button
+            onClick={scanning ? stopScan : startScan}
+            style={{
+              ...btnPrimary,
+              background: scanning ? '#dc2626' : '#3758c9',
+              flexShrink: 0,
+            }}
+          >
+            {scanning ? 'Διακοπή' : 'Σκανάρισμα'}
+          </button>
+        </div>
+
+        {scanning && (
+          <div style={{ padding: '0 24px 12px' }}>
+            <div style={{ height: 6, background: '#f3f4f6', borderRadius: 99, overflow: 'hidden' }}>
+              <div style={{ height: '100%', background: '#3758c9', width: `${pct}%`, transition: 'width 0.3s' }} />
+            </div>
+            <p style={{ fontSize: 11, color: '#9ca3af', marginTop: 4 }}>
+              {progress ? `${progress.done} / ${progress.total} IPs ελέγχθηκαν` : 'Εκκίνηση…'}
+            </p>
+          </div>
+        )}
+
+        {error && (
+          <p style={{ padding: '0 24px 12px', fontSize: 12, color: '#dc2626' }}>{error}</p>
+        )}
+
+        <div style={{ minHeight: 120, maxHeight: 280, overflowY: 'auto', borderTop: '1px solid #f0f1f3' }}>
+          {found.length === 0 && !scanning && (
+            <p style={{ padding: '24px', textAlign: 'center', fontSize: 13, color: '#b8bdc4' }}>
+              {progress ? 'Κανένας εκτυπωτής δεν βρέθηκε.' : 'Πατήστε «Σκανάρισμα» για να ξεκινήσει η αναζήτηση.'}
+            </p>
+          )}
+          {found.map(item => (
+            <div key={item.ip} style={{
+              display: 'flex', alignItems: 'center', justifyContent: 'space-between',
+              padding: '12px 24px', borderBottom: '1px solid #f9fafb',
+            }}>
+              <div>
+                <span style={{ fontSize: 14, fontWeight: 600, color: '#111315' }}>{item.ip}</span>
+                <span style={{ fontSize: 12, color: '#9ca3af', marginLeft: 8 }}>:{item.port}</span>
+              </div>
+              <button
+                onClick={() => { onSelect({ ip_address: item.ip, port: item.port }); onClose() }}
+                style={{ ...btnPrimary, height: 30, padding: '0 14px', fontSize: 12 }}
+              >
+                Χρήση
+              </button>
+            </div>
+          ))}
+        </div>
+
+        <div style={{ padding: '14px 24px', borderTop: '1px solid #f0f1f3', display: 'flex', justifyContent: 'flex-end' }}>
+          <button onClick={onClose} style={btnSecondary}>Κλείσιμο</button>
+        </div>
+      </div>
+    </div>
+  )
+}
+
 function PrintersSection() {
   const qc = useQueryClient()
   const [showNew, setShowNew] = useState(false)
   const [editingId, setEditingId] = useState(null)
+  const [showFinder, setShowFinder] = useState(false)
+  const [newFormInitial, setNewFormInitial] = useState(null)
 
   const { data: printers = [], isLoading } = useQuery({
     queryKey: ['printers-all'],
@@ -810,23 +974,41 @@ function PrintersSection() {
     updateMut.mutate({ id: printer.id, is_active: !printer.is_active })
   }
 
+  function handleFinderSelect(prefill) {
+    setNewFormInitial({ ...EMPTY_FORM, ...prefill })
+    setShowNew(true)
+    setEditingId(null)
+  }
+
   return (
     <div className="card divide-y divide-gray-100">
+      {showFinder && (
+        <FindPrintersModal
+          onClose={() => setShowFinder(false)}
+          onSelect={handleFinderSelect}
+        />
+      )}
       <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'space-between', padding: '16px 20px' }}>
         <div>
           <h2 className="font-semibold text-gray-700">Εκτυπωτές</h2>
           <p className="text-xs text-gray-400 mt-0.5">Διαχείριση εκτυπωτών του συστήματος</p>
         </div>
-        <button onClick={() => { setShowNew(v => !v); setEditingId(null) }} style={btnSecondary}>
+        <div style={{ display: 'flex', gap: 8 }}>
+          <button onClick={() => setShowFinder(true)} style={{ ...btnSecondary, fontSize: 13 }}>
+            Εύρεση εκτυπωτών
+          </button>
+          <button onClick={() => { setNewFormInitial(null); setShowNew(v => !v); setEditingId(null) }} style={btnSecondary}>
             + Νέος εκτυπωτής
           </button>
         </div>
+      </div>
 
       {showNew && (
         <div style={{ padding: '12px 20px' }}>
           <PrinterForm
+            initial={newFormInitial ?? undefined}
             onSave={form => createMut.mutate(form)}
-            onCancel={() => setShowNew(false)}
+            onCancel={() => { setShowNew(false); setNewFormInitial(null) }}
             isPending={createMut.isPending}
           />
         </div>