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>

backend/app/core/duration.py

@@ -0,0 +1,22 @@
+import re
+
+_ISO8601_DURATION_RE = re.compile(
+    r"P(?:(?P<days>\d+)D)?"
+    r"(?:T(?:(?P<hours>\d+)H)?(?:(?P<minutes>\d+)M)?(?:(?P<seconds>\d+)S)?)?"
+)
+
+
+def parse_iso8601_duration(value: str | None) -> int | None:
+    if not value:
+        return None
+    match = _ISO8601_DURATION_RE.fullmatch(value)
+    if not match:
+        return None
+    parts = match.groupdict()
+    if not any(parts.values()):
+        return None
+    days = int(parts["days"] or 0)
+    hours = int(parts["hours"] or 0)
+    minutes = int(parts["minutes"] or 0)
+    seconds = int(parts["seconds"] or 0)
+    return days * 86400 + hours * 3600 + minutes * 60 + seconds