feat: M2 (part 1) — subscription import, YouTube client, quota guard

- Channel/Subscription/Video/ApiQuotaUsage models + migration 0002
- Synchronous YouTube Data API client with OAuth token refresh and per-call
  quota accounting (API key preferred for public reads when configured)
- Subscription import: upsert channels + subscription links, prune unsubscribed,
  fetch channel details (uploads playlist, topics, stats, handle, country)
- Central quota guard tracking units per US-Pacific day against a daily budget
- POST /api/sync/subscriptions and GET /api/sync/status endpoints

backend/app/config.py

@@ -24,6 +24,16 @@ class Settings(BaseSettings):
     # Origin of a separately served frontend dev server (enables CORS). Empty in production.
     frontend_origin: str = ""
 
+    # --- Sync / YouTube Data API ---
+    # Optional API key for public reads (channels/videos/playlistItems). When set it is
+    # preferred for shared backfill/enrichment so it doesn't depend on a specific user.
+    youtube_api_key: str = ""
+    # Daily quota budget (units). Default leaves headroom under the 10,000/day free limit.
+    quota_daily_budget: int = 9000
+    # Recent-first backfill: how far back to fetch on the first pass per channel.
+    backfill_recent_max_videos: int = 200
+    backfill_recent_max_days: int = 365
+
     @property
     def allowed_email_set(self) -> set[str]:
         return {e.strip().lower() for e in self.allowed_emails.split(",") if e.strip()}