feat: queue de téléchargement séquentielle (un à la fois)

asyncio.Queue dans DownloadManager + worker unique démarré dans le lifespan.
Les téléchargements s'exécutent un par un dans l'ordre d'arrivée.
Suppression de BackgroundTasks (plus nécessaire).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

downloader.py

@@ -8,7 +8,6 @@ from datetime import datetime
 from pathlib import Path
 
 import yt_dlp
-from fastapi import BackgroundTasks
 
 OUTPUT_DIR = "/data/Arte"
 DB_PATH = "data/arte_dl.db"
@@ -87,6 +86,7 @@ class DownloadManager:
     def __init__(self):
         self._active: dict[str, dict] = {}
         self._lock = threading.Lock()
+        self._queue: asyncio.Queue = asyncio.Queue()
         self._init_db()
 
     def _init_db(self):
@@ -147,7 +147,8 @@ class DownloadManager:
             ).fetchone()
         return row is not None
 
-    def _insert_queued(self, url: str, title: str) -> str:
+    async def enqueue(self, url: str, title: str, subtitle: str,
+                      year: int | None, category: str) -> str:
         dl_id = str(uuid.uuid4())
         now = datetime.now().isoformat()
         with _db() as conn:
@@ -157,20 +158,15 @@ class DownloadManager:
             )
         with self._lock:
             self._active[dl_id] = {"state": "queued", "progress": 0, "title": title}
+        await self._queue.put((dl_id, url, title, subtitle, year, category))
         return dl_id
 
-    def enqueue(self, url: str, title: str, subtitle: str, year: int | None,
-                category: str, bg: BackgroundTasks) -> str:
-        dl_id = self._insert_queued(url, title)
-        bg.add_task(self._run, dl_id, url, title, subtitle, year, category)
-        return dl_id
-
-    async def enqueue_direct(self, url: str, title: str, subtitle: str,
-                             year: int | None, category: str) -> str:
-        dl_id = self._insert_queued(url, title)
+    async def start_worker(self):
         loop = asyncio.get_running_loop()
-        loop.run_in_executor(None, self._run, dl_id, url, title, subtitle, year, category)
-        return dl_id
+        while True:
+            job = await self._queue.get()
+            dl_id, url, title, subtitle, year, category = job
+            await loop.run_in_executor(None, self._run, dl_id, url, title, subtitle, year, category)
 
     def status(self, dl_id: str) -> dict:
         with self._lock:
@@ -183,13 +179,6 @@ class DownloadManager:
             ).fetchall()
         return [dict(r) for r in rows]
 
-    def already_downloaded(self, url: str) -> bool:
-        with _db() as conn:
-            row = conn.execute(
-                "SELECT id FROM downloads WHERE url=? AND state='done' LIMIT 1", (url,)
-            ).fetchone()
-        return row is not None
-
     # ----------------------------------------------------------------- private
 
     def _set(self, dl_id: str, **kw):