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/metube_client.py

@@ -77,6 +77,21 @@ class MeTubeClient:
         response.raise_for_status()
         return response.json()
 
+    def delete_download(self, metube_job_id: str) -> dict:
+        """Asks MeTube to remove a finished download from its 'done' list and
+        delete the underlying file (actual file deletion additionally depends
+        on MeTube's own DELETE_FILE_ON_TRASHCAN config, which we don't
+        control). Only ever called with a metube_job_id our own app tracked
+        from a download it started -- never touches files MeTube already had
+        before we existed."""
+        response = httpx.post(
+            f"{self.api_base_url}/delete",
+            json={"ids": [metube_job_id], "where": "done"},
+            timeout=self.timeout,
+        )
+        response.raise_for_status()
+        return response.json()
+
     def build_media_url(self, filename: str) -> str | None:
         """Safely turn a MeTube-reported filename into a public /download/... URL.