Fix NFO 503 error on server reload with config fallback

- Add dynamic config loading in get_nfo_service() dependency
- Handle settings reset during uvicorn reload in development
- Add comprehensive tests for settings priority and fallback behavior
- All 4 unit tests passing (settings priority, config fallback, error cases)
- Update documentation with reload scenario fix

src/server/api/nfo.py

@@ -45,14 +45,28 @@ async def get_nfo_service() -> NFOService:
     Raises:
         HTTPException: If NFO service not configured
     """
-    if not settings.tmdb_api_key:
+    # Check if TMDB API key is in settings
+    tmdb_api_key = settings.tmdb_api_key
+    
+    # If not in settings, try to load from config.json
+    if not tmdb_api_key:
+        try:
+            from src.server.services.config_service import get_config_service
+            config_service = get_config_service()
+            config = config_service.load_config()
+            if config.nfo and config.nfo.tmdb_api_key:
+                tmdb_api_key = config.nfo.tmdb_api_key
+        except Exception:
+            pass  # Config loading failed, tmdb_api_key remains None
+    
+    if not tmdb_api_key:
         raise HTTPException(
             status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
             detail="NFO service not configured. TMDB API key required."
         )
     
     return NFOService(
-        tmdb_api_key=settings.tmdb_api_key,
+        tmdb_api_key=tmdb_api_key,
         anime_directory=settings.anime_directory,
         image_size=settings.nfo_image_size,
         auto_create=settings.nfo_auto_create