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.json

@@ -17,7 +17,7 @@
     "keep_days": 30
   },
   "other": {
-    "master_password_hash": "$pbkdf2-sha256$29000$NkbIuZfSGiMEAGDMWau1tg$pMLf3tp4uH36YeG3KBgkX3F00dwbN2dGPQCWNvrjhnU",
+    "master_password_hash": "$pbkdf2-sha256$29000$1TrHeM8Z45yTsjZG6B1DKA$w4AoTXhgcGh90quXesvoRqVkH720fYXEu8LI2L4nUFM",
     "anime_directory": "/mnt/server/serien/Serien/"
   },
   "version": "1.0.0"

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"
+}

src/server/services/anime_service.py

@@ -56,6 +56,8 @@ class AnimeService:
         self._scan_total_items: int = 0
         self._is_scanning: bool = False
         self._scan_current_directory: str = ""
+        # Lock to prevent concurrent rescans
+        self._scan_lock = asyncio.Lock()
         # Subscribe to SeriesApp events
         # Note: Events library uses assignment (=), not += operator
         try:
@@ -482,7 +484,18 @@ class AnimeService:
         
         All series are identified by their 'key' (provider identifier),
         with 'folder' stored as metadata.
+        
+        Note:
+            Only one scan can run at a time. If a scan is already in
+            progress, this method returns immediately without starting
+            a new scan.
         """
+        # Check if a scan is already running (non-blocking)
+        if self._scan_lock.locked():
+            logger.info("Rescan already in progress, ignoring request")
+            return
+        
+        async with self._scan_lock:
             try:
                 # Store event loop for event handlers
                 self._event_loop = asyncio.get_running_loop()