feat: printer duplicate copies setting (Feature 1)

- Add `duplicates` column (0-9) to printers table via migration
- Print loop repeats job 1+duplicates times per printer zone
- PrinterForm in Settings > Print now has ΑΝΤΙΓΡΑΦΑ (0-9) field
- PrinterRow shows amber badge when duplicates > 0

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

local_backend/main.py

@@ -406,6 +406,8 @@ def _run_migrations():
             created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
             priority VARCHAR NOT NULL DEFAULT 'normal'
         )""",
+        # Printer duplicate copies (0 = print once, 1 = print twice, etc.)
+        "ALTER TABLE printers ADD COLUMN duplicates INTEGER NOT NULL DEFAULT 0",
     ]
     for sql in migrations:
         try: