feat: auto cleanup with retention period, storage limit, settings UI

backend/app/main.py

@@ -1,4 +1,5 @@
 """XDL - Twitter/X Video Downloader API."""
+import asyncio
 import os
 from contextlib import asynccontextmanager
 from dotenv import load_dotenv
@@ -9,12 +10,19 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from app.database import init_db
 from app.routes import auth, parse, download, admin
+from app.services.cleanup import cleanup_loop
 
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     await init_db()
+    task = asyncio.create_task(cleanup_loop())
     yield
+    task.cancel()
+    try:
+        await task
+    except asyncio.CancelledError:
+        pass
 
 
 app = FastAPI(title="XDL - Video Downloader", version="1.0.0", lifespan=lifespan)