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_channels.py

@@ -0,0 +1,88 @@
+import pytest
+from fastapi.testclient import TestClient
+
+from app.core.auth_dependency import require_session
+from app.db import get_db
+from app.main import app
+from app.models.channel import Channel
+from app.services import sync
+from app.services.youtube_client import YouTubeInsufficientScope
+
+
+@pytest.fixture
+def client(db_session):
+    def _get_db_override():
+        yield db_session
+
+    app.dependency_overrides[get_db] = _get_db_override
+    app.dependency_overrides[require_session] = lambda: None
+    # Deliberately not using `with TestClient(app)`: that runs the app's
+    # lifespan, which would try to reach the real MeTube instance and DB.
+    yield TestClient(app)
+    del app.dependency_overrides[get_db]
+    del app.dependency_overrides[require_session]
+
+
+def _seed_channel(db_session, youtube_subscription_id="sub1"):
+    channel = Channel(
+        youtube_channel_id="chanA",
+        youtube_subscription_id=youtube_subscription_id,
+        title="Channel A",
+        subscribed=True,
+    )
+    db_session.add(channel)
+    db_session.commit()
+    db_session.refresh(channel)
+    return channel
+
+
+def test_unsubscribe_success(client, db_session, monkeypatch):
+    channel = _seed_channel(db_session)
+
+    monkeypatch.setattr(sync.google_oauth, "get_credentials", lambda db: object())
+    calls = {}
+    monkeypatch.setattr(
+        sync.youtube_client,
+        "unsubscribe",
+        lambda creds, sub_id: calls.setdefault("sub_id", sub_id),
+    )
+
+    resp = client.post(f"/api/channels/{channel.id}/unsubscribe")
+
+    assert resp.status_code == 200
+    assert resp.json()["subscribed"] is False
+    assert calls["sub_id"] == "sub1"
+
+    db_session.refresh(channel)
+    assert channel.subscribed is False
+
+
+def test_unsubscribe_without_subscription_id_returns_400(client, db_session):
+    channel = _seed_channel(db_session, youtube_subscription_id=None)
+
+    resp = client.post(f"/api/channels/{channel.id}/unsubscribe")
+
+    assert resp.status_code == 400
+
+
+def test_unsubscribe_insufficient_scope_returns_403(client, db_session, monkeypatch):
+    channel = _seed_channel(db_session)
+
+    monkeypatch.setattr(sync.google_oauth, "get_credentials", lambda db: object())
+
+    def raise_insufficient(creds, sub_id):
+        raise YouTubeInsufficientScope("Request had insufficient authentication scopes.")
+
+    monkeypatch.setattr(sync.youtube_client, "unsubscribe", raise_insufficient)
+
+    resp = client.post(f"/api/channels/{channel.id}/unsubscribe")
+
+    assert resp.status_code == 403
+
+    db_session.refresh(channel)
+    assert channel.subscribed is True
+
+
+def test_unsubscribe_channel_not_found(client):
+    resp = client.post("/api/channels/9999/unsubscribe")
+    assert resp.status_code == 404