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/sync.py

@@ -78,6 +78,7 @@ def sync_subscriptions(db: Session) -> dict:
             if channel is None:
                 channel = Channel(
                     youtube_channel_id=sub["youtube_channel_id"],
+                    youtube_subscription_id=sub["youtube_subscription_id"],
                     title=sub["title"],
                     description=sub["description"],
                     thumbnail_url=sub["thumbnail_url"],
@@ -88,6 +89,7 @@ def sync_subscriptions(db: Session) -> dict:
                 existing[sub["youtube_channel_id"]] = channel
                 added += 1
             else:
+                channel.youtube_subscription_id = sub["youtube_subscription_id"]
                 channel.title = sub["title"]
                 channel.description = sub["description"]
                 channel.thumbnail_url = sub["thumbnail_url"]
@@ -249,3 +251,21 @@ def sync_videos(db: Session) -> dict:
         raise
     finally:
         _videos_lock.release()
+
+
+class ChannelHasNoSubscriptionId(Exception):
+    pass
+
+
+def unsubscribe_channel(db: Session, channel: Channel) -> None:
+    """Actually unsubscribes on YouTube (subscriptions.delete) -- deliberately
+    out of the original MVP scope, added on explicit user request. Requires
+    the write-capable OAuth scope; see google_oauth.SCOPES."""
+    if not channel.youtube_subscription_id:
+        raise ChannelHasNoSubscriptionId(
+            "This channel was synced before subscription ids were tracked; run a subscriptions sync first"
+        )
+    credentials = google_oauth.get_credentials(db)
+    youtube_client.unsubscribe(credentials, channel.youtube_subscription_id)
+    channel.subscribed = False
+    db.commit()