Replace asyncio.to_thread with ThreadPoolExecutor.run_in_executor

- Add ThreadPoolExecutor with 3 max workers to SeriesApp
- Replace all asyncio.to_thread calls with loop.run_in_executor
- Add shutdown() method to properly cleanup executor
- Integrate SeriesApp.shutdown() into FastAPI shutdown sequence
- Ensures proper resource cleanup on Ctrl+C (SIGINT/SIGTERM)

src/server/fastapi_app.py

@@ -197,7 +197,17 @@ async def lifespan(_application: FastAPI):
     except Exception as e:  # pylint: disable=broad-exception-caught
         logger.error("Error stopping download service: %s", e, exc_info=True)
     
-    # 3. Cleanup progress service
+    # 3. Shutdown SeriesApp and cleanup thread pool
+    try:
+        from src.server.utils.dependencies import _series_app
+        if _series_app is not None:
+            logger.info("Shutting down SeriesApp thread pool...")
+            _series_app.shutdown()
+            logger.info("SeriesApp shutdown complete")
+    except Exception as e:  # pylint: disable=broad-exception-caught
+        logger.error("Error during SeriesApp shutdown: %s", e, exc_info=True)
+    
+    # 4. Cleanup progress service
     try:
         progress_service = get_progress_service()
         logger.info("Cleaning up progress service...")
@@ -205,9 +215,11 @@ async def lifespan(_application: FastAPI):
         progress_service._active_progress.clear()
         logger.info("Progress service cleanup complete")
     except Exception as e:  # pylint: disable=broad-exception-caught
-        logger.error("Error cleaning up progress service: %s", e, exc_info=True)
+        logger.error(
+            "Error cleaning up progress service: %s", e, exc_info=True
+        )
     
-    # 4. Close database connections with WAL checkpoint
+    # 5. Close database connections with WAL checkpoint
     try:
         from src.server.database.connection import close_db
         logger.info("Closing database connections...")