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/google_oauth.py

@@ -14,7 +14,13 @@ from app.models.oauth_credentials import SINGLETON_ID, OAuthCredentials
 logger = logging.getLogger(__name__)
 
 SCOPES = [
-    "https://www.googleapis.com/auth/youtube.readonly",
+    # Full read/write scope, not just youtube.readonly: unsubscribing from a
+    # channel (subscriptions.delete) requires write access. Deliberate
+    # deviation from the original "minimal scope" TZ recommendation, per
+    # explicit user request. Existing stored refresh tokens were granted
+    # under the old readonly-only scope and won't cover this -- users must
+    # reconnect once for this to take effect.
+    "https://www.googleapis.com/auth/youtube",
     "openid",
     "https://www.googleapis.com/auth/userinfo.email",
     "https://www.googleapis.com/auth/userinfo.profile",