fix(mqtt): stop logging retained boot_report/alerts as new events

The firmware publishes boot_report on system/info and alerts on
system/alerts with retain=true. Every time the backend (re)connects - which
under uvicorn --reload means every backend file save - the broker redelivers
the last retained message and we inserted it again with occurred_at=now().
Result: the Health tab showed fresh PANIC boots and "Device reset due to
fault" alerts for a device that had been up for 4 days.

- insert_boot_event skips the insert when a row with the same
  (device_serial, boot_count) already exists. boot_count is the firmware's
  lifetime counter, so it uniquely identifies a boot.
- upsert_alert only writes when state/message actually changed and returns
  whether it did; the alert-event history row is only added on a change.
  A redelivered identical alert no longer bumps updated_at either.

Existing duplicate rows are not touched by this commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/database/pg_mqtt.py

@@ -300,9 +300,13 @@ async def get_pending_command(device_serial: str) -> dict | None:
 # ---------------------------------------------------------------------------
 
 async def upsert_alert(device_serial: str, subsystem: str, state: str,
-                       message: str | None = None):
+                       message: str | None = None) -> bool:
+    """Insert or update the current alert. Returns False (and leaves the row,
+    including updated_at, untouched) when it already holds this exact state and
+    message — which is what a retained system/alerts message looks like when the
+    broker redelivers it on every backend (re)subscribe."""
     async with AsyncSessionLocal() as session:
-        await session.execute(
+        result = await session.execute(
             text("""
                 INSERT INTO device_alerts (device_serial, subsystem, state, message, updated_at)
                 VALUES (:serial, :subsystem, :state, :message, now())
@@ -311,10 +315,15 @@ async def upsert_alert(device_serial: str, subsystem: str, state: str,
                     state      = EXCLUDED.state,
                     message    = EXCLUDED.message,
                     updated_at = EXCLUDED.updated_at
+                WHERE device_alerts.state   IS DISTINCT FROM EXCLUDED.state
+                   OR device_alerts.message IS DISTINCT FROM EXCLUDED.message
+                RETURNING id
             """),
             {"serial": device_serial, "subsystem": subsystem, "state": state, "message": message},
         )
+        changed = result.fetchone() is not None
         await session.commit()
+    return changed
 
 
 async def delete_alert(device_serial: str, subsystem: str):
@@ -435,8 +444,24 @@ async def insert_boot_event(device_serial: str, boot_count: int | None,
                             crash_task: str | None = None,
                             crash_pc: int | None = None,
                             crash_exc_cause: int | None = None,
-                            crash_exc_vaddr: int | None = None) -> int:
+                            crash_exc_vaddr: int | None = None) -> int | None:
+    """Insert a boot event, unless one with the same boot_count already exists
+    for this device (returns None then). boot_report is published retained on
+    system/info, so the broker redelivers the last one every time the backend
+    (re)subscribes — without this check each backend restart logged the
+    device's last boot (crash detail included) again as a brand-new event."""
     async with AsyncSessionLocal() as session:
+        if boot_count is not None:
+            dup = await session.execute(
+                text("""
+                    SELECT 1 FROM device_boot_events
+                    WHERE device_serial = :serial AND boot_count = :boot_count
+                    LIMIT 1
+                """),
+                {"serial": device_serial, "boot_count": boot_count},
+            )
+            if dup.first() is not None:
+                return None
         result = await session.execute(
             text("""
                 INSERT INTO device_boot_events
@@ -461,7 +486,7 @@ async def insert_boot_event(device_serial: str, boot_count: int | None,
         )
         row = result.fetchone()
         await session.commit()
-        return row[0]
+        return row[0] if row else None
 
 
 async def get_boot_events(device_serial: str, limit: int = 100, offset: int = 0,

backend/mqtt/logger.py

@@ -86,9 +86,12 @@ async def _handle_alerts(serial: str, payload: dict):
     if state == "CLEARED":
         await db.delete_alert(serial, subsystem)
     else:
-        await db.upsert_alert(serial, subsystem, state, payload.get("msg"))
+        changed = await db.upsert_alert(serial, subsystem, state, payload.get("msg"))
         # Append-only history — survives past the alert being resolved, used to
         # answer "when was the most recent issue" even once it's cleared.
+        # Skipped when nothing changed: alerts are retained, so the broker
+        # re-sends every active one on each backend (re)subscribe.
+        if changed:
             await db.insert_alert_event(serial, subsystem, state, payload.get("msg"))