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_metube_client.py

@@ -0,0 +1,33 @@
+import pytest
+
+from app.services.metube_client import MeTubeClient
+
+
+@pytest.fixture
+def client():
+    return MeTubeClient()
+
+
+def test_build_media_url_encodes_spaces(client):
+    url = client.build_media_url("some file.mp4")
+    assert url == f"{client.public_base_url}/download/some%20file.mp4"
+
+
+def test_build_media_url_strips_absolute_download_dir_prefix(client):
+    filename = f"{client.download_dir}/nested/video.mp4"
+    url = client.build_media_url(filename)
+    assert url == f"{client.public_base_url}/download/nested/video.mp4"
+
+
+def test_build_media_url_rejects_path_traversal(client):
+    assert client.build_media_url("../../etc/passwd") is None
+
+
+def test_build_media_url_none_for_empty(client):
+    assert client.build_media_url("") is None
+    assert client.build_media_url(None) is None
+
+
+def test_build_media_url_encodes_each_segment(client):
+    url = client.build_media_url("dir with space/file#1.mp4")
+    assert url == f"{client.public_base_url}/download/dir%20with%20space/file%231.mp4"