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/quota.py

@@ -0,0 +1,50 @@
+"""Central YouTube Data API quota guard.
+
+The whole app shares one daily budget (the API resets at midnight US Pacific time).
+Steady-state work (RSS detection is free; enrichment of new videos is cheap) should
+always be allowed; expensive backfill must yield to the remaining budget.
+"""
+from datetime import datetime
+from zoneinfo import ZoneInfo
+
+from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.orm import Session
+
+from app.config import settings
+from app.models import ApiQuotaUsage
+
+_PACIFIC = ZoneInfo("America/Los_Angeles")
+
+
+def pacific_today():
+    return datetime.now(_PACIFIC).date()
+
+
+def units_used_today(db: Session) -> int:
+    row = db.get(ApiQuotaUsage, pacific_today())
+    return row.units_used if row else 0
+
+
+def remaining_today(db: Session) -> int:
+    return max(0, settings.quota_daily_budget - units_used_today(db))
+
+
+def can_spend(db: Session, units: int) -> bool:
+    return remaining_today(db) >= units
+
+
+def record_usage(db: Session, units: int) -> None:
+    """Atomically add `units` to today's counter (upsert)."""
+    if units <= 0:
+        return
+    day = pacific_today()
+    stmt = (
+        pg_insert(ApiQuotaUsage)
+        .values(day=day, units_used=units)
+        .on_conflict_do_update(
+            index_elements=[ApiQuotaUsage.day],
+            set_={"units_used": ApiQuotaUsage.units_used + units},
+        )
+    )
+    db.execute(stmt)
+    db.commit()