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/models/channel_category.py

@@ -0,0 +1,10 @@
+from sqlalchemy import Column, ForeignKey, Integer, Table
+
+from app.db import Base
+
+channel_categories = Table(
+    "channel_categories",
+    Base.metadata,
+    Column("channel_id", Integer, ForeignKey("channels.id"), primary_key=True),
+    Column("category_id", Integer, ForeignKey("categories.id", ondelete="CASCADE"), primary_key=True),
+)