Implement Phase 6: MeTube download integration

- MeTubeClient encapsulates all MeTube HTTP/Socket.IO calls (verified against
  the real MeTube source: /add returns no job id, GET /history gives a queue
  snapshot for reconciliation, filenames arrive already relative, percent is
  a 0-100 float)
- download_jobs table + service: request/dedup active downloads, apply live
  Socket.IO events (added/updated/completed/canceled/cleared) matched by
  canonical YouTube URL, safe relative-path -> public media URL construction
- Reconciliation on startup against MeTube's live queue/done state (section 19):
  non-terminal jobs recovered where possible, else marked "unknown"; already
  completed jobs are left untouched
- POST/GET /api/videos/{id}/download(-status), recheck-local; feed/video
  detail now report real local availability instead of a stub
- Frontend: download button with live status polling (queued/downloading %/
  postprocessing/completed/failed+retry), local <video> playback with
  YouTube fallback on playback error
- health.py now delegates to MeTubeClient (single place for MeTube calls)

26 new backend tests (63 total). Verified live: Socket.IO connects
successfully to the real MeTube instance on deploy.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_videos.py

@@ -19,8 +19,10 @@ def client(db_session):
 
     app.dependency_overrides[get_db] = _get_db_override
     app.dependency_overrides[require_session] = lambda: None
-    with TestClient(app) as test_client:
-        yield test_client
+    # Deliberately not using `with TestClient(app)`: that runs the app's
+    # lifespan, which would try to reach the real MeTube instance and DB
+    # (see AGENTS.md: MeTube network calls in tests must be opt-in only).
+    yield TestClient(app)
     del app.dependency_overrides[get_db]
     del app.dependency_overrides[require_session]
 
@@ -60,3 +62,99 @@ def test_get_video_by_youtube_id(client, db_session):
 def test_get_video_not_found(client):
     resp = client.get("/api/videos/missing")
     assert resp.status_code == 404
+
+
+def _seed_single_video(db_session, youtube_video_id="vid1"):
+    channel = Channel(youtube_channel_id="chanA", title="Channel A", subscribed=True)
+    db_session.add(channel)
+    db_session.commit()
+
+    video = Video(
+        youtube_video_id=youtube_video_id,
+        channel_id=channel.id,
+        title="Video One",
+        published_at=datetime(2026, 9, 10, tzinfo=timezone.utc),
+        youtube_url=f"https://www.youtube.com/watch?v={youtube_video_id}",
+    )
+    db_session.add(video)
+    db_session.commit()
+    db_session.refresh(video)
+    return video
+
+
+def test_download_video_enqueues(client, db_session, monkeypatch):
+    video = _seed_single_video(db_session)
+    monkeypatch.setattr(
+        "app.services.metube_client.MeTubeClient.enqueue_video", lambda self, u, p: {"status": "ok"}
+    )
+
+    resp = client.post(f"/api/videos/{video.youtube_video_id}/download")
+
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "queued"
+
+
+def test_download_video_metube_rejects(client, db_session, monkeypatch):
+    video = _seed_single_video(db_session)
+    monkeypatch.setattr(
+        "app.services.metube_client.MeTubeClient.enqueue_video",
+        lambda self, u, p: {"status": "error", "msg": "bad url"},
+    )
+
+    resp = client.post(f"/api/videos/{video.youtube_video_id}/download")
+
+    assert resp.status_code == 502
+
+
+def test_download_video_second_click_returns_same_job(client, db_session, monkeypatch):
+    video = _seed_single_video(db_session)
+    monkeypatch.setattr(
+        "app.services.metube_client.MeTubeClient.enqueue_video", lambda self, u, p: {"status": "ok"}
+    )
+
+    r1 = client.post(f"/api/videos/{video.youtube_video_id}/download").json()
+    r2 = client.post(f"/api/videos/{video.youtube_video_id}/download").json()
+
+    assert r1["requested_at"] == r2["requested_at"]
+
+    from app.models.download_job import DownloadJob
+
+    assert db_session.query(DownloadJob).count() == 1
+
+
+def test_download_status_not_downloaded(client, db_session):
+    video = _seed_single_video(db_session)
+    resp = client.get(f"/api/videos/{video.youtube_video_id}/download-status")
+    assert resp.json()["status"] == "not_downloaded"
+
+
+def test_recheck_local_downgrades_to_unknown_when_media_missing(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", media_url="http://metube.local/download/f.mp4")
+    db_session.add(job)
+    db_session.commit()
+
+    monkeypatch.setattr("app.services.metube_client.MeTubeClient.check_media", lambda self, url: False)
+
+    resp = client.post(f"/api/videos/{video.youtube_video_id}/recheck-local")
+
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "unknown"
+
+
+def test_recheck_local_keeps_completed_when_media_reachable(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", media_url="http://metube.local/download/f.mp4")
+    db_session.add(job)
+    db_session.commit()
+
+    monkeypatch.setattr("app.services.metube_client.MeTubeClient.check_media", lambda self, url: True)
+
+    resp = client.post(f"/api/videos/{video.youtube_video_id}/recheck-local")
+
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "completed"