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/api/health.py

@@ -0,0 +1,42 @@
+import logging
+
+import httpx
+from fastapi import APIRouter, Depends
+from sqlalchemy import text
+from sqlalchemy.orm import Session
+
+from app.config import settings
+from app.db import get_db
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter()
+
+
+def _check_database(db: Session) -> str:
+    try:
+        db.execute(text("SELECT 1"))
+        return "ok"
+    except Exception:
+        logger.exception("Database healthcheck failed")
+        return "error"
+
+
+def _check_metube() -> str:
+    try:
+        response = httpx.get(settings.metube_api_base_url, timeout=5)
+        if response.status_code < 500:
+            return "ok"
+        return "error"
+    except Exception:
+        logger.warning("MeTube healthcheck failed", exc_info=True)
+        return "error"
+
+
+@router.get("/health")
+def health(db: Session = Depends(get_db)) -> dict:
+    return {
+        "status": "ok",
+        "database": _check_database(db),
+        "metube": _check_metube(),
+    }