Prevent concurrent rescans with async lock

- Add _scan_lock asyncio.Lock to AnimeService
- Check if lock is held before starting rescan
- Use async with to ensure lock is released on completion or exception
- All 1024 tests passing

data/config_backups/config_backup_20251224_210928.json

@@ -0,0 +1,24 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$tTYmJCSEUArhnLN2TmktpQ$mhMKshEetPJfjRqYYUUvOGNRxcnIMNIto73IRKw4hPM",
+    "anime_directory": "/mnt/server/serien/Serien/"
+  },
+  "version": "1.0.0"
+}