feat: implement graceful shutdown with SIGINT/SIGTERM support

- Add WebSocket shutdown() with client notification and graceful close
- Enhance download service stop() with pending state persistence
- Expand FastAPI lifespan shutdown with proper cleanup sequence
- Add SQLite WAL checkpoint before database close
- Update stop_server.sh to use SIGTERM with timeout fallback
- Configure uvicorn timeout_graceful_shutdown=30s
- Update ARCHITECTURE.md with shutdown documentation

run_server.py

@@ -2,7 +2,8 @@
 """
 Startup script for the Aniworld FastAPI application.
 
-This script starts the application with proper logging configuration.
+This script starts the application with proper logging configuration
+and graceful shutdown support via Ctrl+C (SIGINT) or SIGTERM.
 """
 import uvicorn
 
@@ -15,6 +16,11 @@ if __name__ == "__main__":
     # Run the application with logging.
     # Only watch .py files in src/, explicitly exclude __pycache__.
     # This prevents reload loops from .pyc compilation.
+    #
+    # Graceful shutdown:
+    # - Ctrl+C (SIGINT) or SIGTERM triggers graceful shutdown
+    # - timeout_graceful_shutdown ensures shutdown completes within 30s
+    # - The FastAPI lifespan handler orchestrates cleanup in proper order
     uvicorn.run(
         "src.server.fastapi_app:app",
         host="127.0.0.1",
@@ -24,4 +30,5 @@ if __name__ == "__main__":
         reload_includes=["*.py"],
         reload_excludes=["*/__pycache__/*", "*.pyc"],
         log_config=log_config,
+        timeout_graceful_shutdown=30,  # Allow 30s for graceful shutdown
     )