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>

tests/test_videos.py

@@ -158,3 +158,40 @@ def test_recheck_local_keeps_completed_when_media_reachable(client, db_session,
 
     assert resp.status_code == 200
     assert resp.json()["status"] == "completed"
+
+
+def test_delete_download_requires_completed_job(client, db_session):
+    video = _seed_single_video(db_session)
+    resp = client.delete(f"/api/videos/{video.youtube_video_id}/download")
+    assert resp.status_code == 400
+
+
+def test_delete_download_success(client, db_session, monkeypatch):
+    from app.models.download_job import DownloadJob
+
+    video = _seed_single_video(db_session)
+    job = DownloadJob(
+        video_id=video.id,
+        status="completed",
+        metube_job_id="vid1.vid1",
+        media_url="http://metube.local/download/f.mp4",
+    )
+    db_session.add(job)
+    db_session.commit()
+
+    calls = {}
+    monkeypatch.setattr(
+        "app.services.metube_client.MeTubeClient.delete_download",
+        lambda self, metube_job_id: calls.setdefault("id", metube_job_id) or {"status": "ok"},
+    )
+
+    resp = client.delete(f"/api/videos/{video.youtube_video_id}/download")
+
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "deleted"
+    assert resp.json()["media_url"] is None
+    assert calls["id"] == "vid1.vid1"
+
+    db_session.refresh(job)
+    assert job.status == "deleted"
+    assert job.media_url is None