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>

Dockerfile

@@ -0,0 +1,30 @@
+FROM node:22-slim AS frontend-build
+WORKDIR /frontend
+COPY frontend/package.json frontend/package-lock.json ./
+RUN npm ci
+COPY frontend/ ./
+RUN npm run build
+
+FROM python:3.13-slim AS backend
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY backend/requirements.txt ./backend/requirements.txt
+RUN pip install --no-cache-dir -r backend/requirements.txt
+
+COPY backend/ ./backend/
+COPY migrations/ ./migrations/
+COPY alembic.ini ./alembic.ini
+COPY --from=frontend-build /frontend/dist ./backend/static
+
+ENV PYTHONPATH=/app/backend
+WORKDIR /app
+
+COPY entrypoint.sh ./entrypoint.sh
+RUN chmod +x ./entrypoint.sh
+
+EXPOSE 8080
+
+ENTRYPOINT ["./entrypoint.sh"]