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

src/server/database/connection.py

@@ -150,11 +150,29 @@ async def init_db() -> None:
 async def close_db() -> None:
     """Close database connections and cleanup resources.
     
+    Performs a WAL checkpoint for SQLite databases to ensure all
+    pending writes are flushed to the main database file before
+    closing connections. This prevents database corruption during
+    shutdown.
+    
     Should be called during application shutdown.
     """
     global _engine, _sync_engine, _session_factory, _sync_session_factory
     
     try:
+        # For SQLite: checkpoint WAL to ensure all writes are flushed
+        if _sync_engine and "sqlite" in str(_sync_engine.url):
+            logger.info("Running SQLite WAL checkpoint before shutdown...")
+            try:
+                from sqlalchemy import text
+                with _sync_engine.connect() as conn:
+                    # TRUNCATE mode: checkpoint and truncate WAL file
+                    conn.execute(text("PRAGMA wal_checkpoint(TRUNCATE)"))
+                    conn.commit()
+                logger.info("SQLite WAL checkpoint completed")
+            except Exception as e:
+                logger.warning(f"WAL checkpoint failed (non-critical): {e}")
+        
         if _engine:
             logger.info("Closing async database engine...")
             await _engine.dispose()