Add delete-downloaded-video and real YouTube unsubscribe

Delete local copy:
- MeTubeClient.delete_download() -> POST /delete {ids, where: "done"}
- Only ever acts on a job our own app tracked (metube_job_id we stored from
  its own 'completed' event), never a pre-existing MeTube file
- New "deleted" terminal status; DELETE /api/videos/{id}/download
- Frontend: delete button next to "На сервере" badge, confirm dialog

Unsubscribe (deliberate deviation from the original TZ's MVP exclusion of
subscription management, per explicit user request after being shown the
tradeoff):
- OAuth scope widened from youtube.readonly to full youtube (read/write) --
  existing stored tokens only cover the old scope, so unsubscribing needs a
  fresh reconnect; reads keep working unchanged on the old token meanwhile
- channels.youtube_subscription_id (distinct from the channel id; that's
  what subscriptions.delete actually keys on) captured during subscriptions
  sync
- YouTubeInsufficientScope raised on 401/403 "insufficient authentication
  scopes" and surfaced as a clear 403 asking the user to reconnect, rather
  than a generic API error
- POST /api/channels/{id}/unsubscribe calls subscriptions.delete and marks
  the channel unsubscribed locally on success
- Frontend: "Отписаться" button on ChannelCard with confirm dialog

10 new backend tests (73 total).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/api/videos.py

@@ -8,7 +8,13 @@ from app.db import get_db
 from app.models.channel import Channel
 from app.models.download_job import DownloadJob
 from app.models.video import Video
-from app.services.download_jobs import MeTubeRejected, get_latest_job, request_download
+from app.services.download_jobs import (
+    DeleteNotAllowed,
+    MeTubeRejected,
+    delete_local_copy,
+    get_latest_job,
+    request_download,
+)
 from app.services.metube_client import MeTubeClient
 from app.services.video_presentation import channel_categories_map, serialize_video
 
@@ -64,6 +70,21 @@ def download_video(youtube_video_id: str, db: Session = Depends(get_db)) -> dict
     return _serialize_job(job)
 
 
+@router.delete("/videos/{youtube_video_id}/download")
+def delete_download(youtube_video_id: str, db: Session = Depends(get_db)) -> dict:
+    video = _get_video_or_404(db, youtube_video_id)
+
+    try:
+        job = delete_local_copy(db, video)
+    except DeleteNotAllowed as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    except Exception:
+        logger.exception("Failed to delete local copy for %s", youtube_video_id)
+        raise HTTPException(status_code=502, detail="MeTube is unavailable")
+
+    return _serialize_job(job)
+
+
 @router.get("/videos/{youtube_video_id}/download-status")
 def download_status(youtube_video_id: str, db: Session = Depends(get_db)) -> dict:
     video = _get_video_or_404(db, youtube_video_id)