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

docs/ARCHITECTURE.md

@@ -141,6 +141,78 @@ Source: [src/config/settings.py](../src/config/settings.py#L1-L96)
 
 ---
 
+## 11. Graceful Shutdown
+
+The application implements a comprehensive graceful shutdown mechanism that ensures data integrity and proper cleanup when the server is stopped via Ctrl+C (SIGINT) or SIGTERM.
+
+### 11.1 Shutdown Sequence
+
+```
+1. SIGINT/SIGTERM received
+   +-- Uvicorn catches signal
+   +-- Stops accepting new requests
+
+2. FastAPI lifespan shutdown triggered
+   +-- 30 second total timeout
+
+3. WebSocket shutdown (5s timeout)
+   +-- Broadcast {"type": "server_shutdown"} to all clients
+   +-- Close each connection with code 1001 (Going Away)
+   +-- Clear connection tracking data
+
+4. Download service stop (10s timeout)
+   +-- Set shutdown flag
+   +-- Persist active download as "pending" in database
+   +-- Cancel active download task
+   +-- Shutdown ThreadPoolExecutor with wait
+
+5. Progress service cleanup
+   +-- Clear event subscribers
+   +-- Clear active progress tracking
+
+6. Database cleanup (10s timeout)
+   +-- SQLite: Run PRAGMA wal_checkpoint(TRUNCATE)
+   +-- Dispose async engine
+   +-- Dispose sync engine
+
+7. Process exits cleanly
+```
+
+Source: [src/server/fastapi_app.py](../src/server/fastapi_app.py#L142-L210)
+
+### 11.2 Key Components
+
+| Component           | File                                                                                       | Shutdown Method          |
+| ------------------- | ------------------------------------------------------------------------------------------ | ------------------------ |
+| WebSocket Service   | [websocket_service.py](../src/server/services/websocket_service.py)                        | `shutdown(timeout=5.0)`  |
+| Download Service    | [download_service.py](../src/server/services/download_service.py)                          | `stop(timeout=10.0)`     |
+| Database Connection | [connection.py](../src/server/database/connection.py)                                      | `close_db()`             |
+| Uvicorn Config      | [run_server.py](../run_server.py)                                                          | `timeout_graceful_shutdown=30` |
+| Stop Script         | [stop_server.sh](../stop_server.sh)                                                        | SIGTERM with fallback    |
+
+### 11.3 Data Integrity Guarantees
+
+1. **Active downloads preserved**: In-progress downloads are saved as "pending" and can resume on restart.
+
+2. **Database WAL flushed**: SQLite WAL checkpoint ensures all writes are in the main database file.
+
+3. **WebSocket clients notified**: Clients receive shutdown message before connection closes.
+
+4. **Thread pool cleanup**: Background threads complete or are gracefully cancelled.
+
+### 11.4 Manual Stop
+
+```bash
+# Graceful stop via script (sends SIGTERM, waits up to 30s)
+./stop_server.sh
+
+# Or press Ctrl+C in terminal running the server
+```
+
+Source: [stop_server.sh](../stop_server.sh#L1-L80)
+
+---
+
 ## 3. Component Interactions
 
 ### 3.1 Request Flow (REST API)