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/services/download_jobs.py

@@ -15,6 +15,10 @@ class MeTubeRejected(Exception):
     pass
 
 
+class DeleteNotAllowed(Exception):
+    pass
+
+
 def get_latest_job(db: Session, video_id: int) -> DownloadJob | None:
     return (
         db.query(DownloadJob)
@@ -55,6 +59,24 @@ def request_download(db: Session, video: Video) -> DownloadJob:
     return job
 
 
+def delete_local_copy(db: Session, video: Video) -> DownloadJob:
+    """Deletes our own previously-downloaded copy from MeTube. Only ever acts
+    on a job this app itself created and tracked (never a file MeTube already
+    had before we existed) -- see AGENTS.md constraint 10."""
+    job = get_latest_job(db, video.id)
+    if job is None or job.status != "completed":
+        raise DeleteNotAllowed("No completed local copy to delete")
+    if not job.metube_job_id:
+        raise DeleteNotAllowed("Missing MeTube job id, cannot request deletion")
+
+    MeTubeClient().delete_download(job.metube_job_id)
+
+    job.status = "deleted"
+    job.media_url = None
+    db.commit()
+    return job
+
+
 def _find_job_by_payload(db: Session, payload: dict) -> DownloadJob | None:
     metube_id = payload.get("id")
     if metube_id: