feat: dedup downloads by (url, format_id) index, reuse existing files

backend/app/routes/download.py

@@ -50,6 +50,19 @@ async def _do_download(task_id: str, url: str, format_id: str):
 
 @router.post("/download", response_model=DownloadResponse)
 async def start_download(req: DownloadRequest, background_tasks: BackgroundTasks, db: AsyncSession = Depends(get_db)):
+    # Dedup: reuse existing completed download if file still on disk
+    existing = (await db.execute(
+        select(Video).where(
+            Video.url == req.url,
+            Video.format_id == req.format_id,
+            Video.status == "done",
+        ).order_by(Video.created_at.desc()).limit(1)
+    )).scalar_one_or_none()
+
+    if existing and os.path.exists(existing.file_path):
+        logger.info(f"Reusing existing download task_id={existing.task_id} for url={req.url} format={req.format_id}")
+        return DownloadResponse(task_id=existing.task_id, status="done")
+
     task_id = str(uuid.uuid4())[:8]
     video = Video(task_id=task_id, url=req.url, quality=req.quality, format_id=req.format_id, status="pending")
     db.add(video)