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_youtube_client.py

@@ -40,12 +40,13 @@ def test_fetch_subscriptions_paginates(monkeypatch):
         {
             "items": [
                 {
+                    "id": "subA",
                     "snippet": {
                         "title": "Channel A",
                         "description": "desc a",
                         "resourceId": {"channelId": "chanA"},
                         "thumbnails": {"high": {"url": "http://thumb/a.jpg"}},
-                    }
+                    },
                 }
             ],
             "nextPageToken": "page2",
@@ -72,6 +73,7 @@ def test_fetch_subscriptions_paginates(monkeypatch):
     result = youtube_client.fetch_subscriptions(_credentials())
 
     assert [r["youtube_channel_id"] for r in result] == ["chanA", "chanB"]
+    assert result[0]["youtube_subscription_id"] == "subA"
     assert result[0]["thumbnail_url"] == "http://thumb/a.jpg"
     assert result[1]["thumbnail_url"] == "http://thumb/b.jpg"
 
@@ -143,6 +145,50 @@ def test_fetch_videos_details(monkeypatch):
     assert result[0]["thumbnail_url"] == "http://thumb/1.jpg"
 
 
+def test_unsubscribe_success(monkeypatch):
+    response = FakeResponse(204, {})
+    seen = {}
+
+    class FakeDeleteClient(FakeClient):
+        def delete(self, url, params=None, headers=None):
+            seen["params"] = params
+            return self._responses.pop(0)
+
+    monkeypatch.setattr(youtube_client.httpx, "Client", lambda timeout: FakeDeleteClient([response]))
+
+    youtube_client.unsubscribe(_credentials(), "sub123")
+
+    assert seen["params"] == {"id": "sub123"}
+
+
+def test_unsubscribe_already_gone_is_not_an_error(monkeypatch):
+    response = FakeResponse(404, {"error": {"message": "not found"}})
+
+    class FakeDeleteClient(FakeClient):
+        def delete(self, url, params=None, headers=None):
+            return self._responses.pop(0)
+
+    monkeypatch.setattr(youtube_client.httpx, "Client", lambda timeout: FakeDeleteClient([response]))
+
+    youtube_client.unsubscribe(_credentials(), "sub123")  # must not raise
+
+
+def test_unsubscribe_insufficient_scope(monkeypatch):
+    response = FakeResponse(
+        403,
+        {"error": {"message": "Request had insufficient authentication scopes.", "errors": [{"reason": "forbidden"}]}},
+    )
+
+    class FakeDeleteClient(FakeClient):
+        def delete(self, url, params=None, headers=None):
+            return self._responses.pop(0)
+
+    monkeypatch.setattr(youtube_client.httpx, "Client", lambda timeout: FakeDeleteClient([response]))
+
+    with pytest.raises(youtube_client.YouTubeInsufficientScope):
+        youtube_client.unsubscribe(_credentials(), "sub123")
+
+
 def test_fetch_uploads_playlists_batches(monkeypatch):
     response = FakeResponse(
         200,