Implement Phases 1-5: skeleton, OAuth, categories, video sync/feed, playback

- FastAPI + PostgreSQL + Alembic + React/Vite skeleton, Docker Compose, healthcheck
- Google OAuth (single allowed account), encrypted refresh token storage
- Subscriptions sync with pagination, uploads playlist batch fetch
- Categories CRUD, many-to-many channel assignment, category filtering
- Video sync (playlistItems + videos.list batching), cached feed with cursor
  pagination, background scheduler (APScheduler)
- Video detail page with YouTube embed player
- SPA fallback routing, optimistic UI updates, client-side query caching

40 backend tests covering OAuth allow-list, sync idempotency, cascade deletes,
cursor pagination, and category filtering.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_duration.py

@@ -0,0 +1,20 @@
+import pytest
+
+from app.core.duration import parse_iso8601_duration
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        ("PT18M42S", 18 * 60 + 42),
+        ("PT1H2M3S", 3600 + 120 + 3),
+        ("PT45S", 45),
+        ("PT5M", 300),
+        ("P1DT2H", 86400 + 7200),
+        (None, None),
+        ("", None),
+        ("garbage", None),
+    ],
+)
+def test_parse_iso8601_duration(value, expected):
+    assert parse_iso8601_duration(value) == expected