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/routes/sync.py

@@ -0,0 +1,40 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session
+
+from app import quota
+from app.auth import current_user
+from app.db import get_db
+from app.models import Channel, Subscription, User, Video
+from app.sync.subscriptions import import_subscriptions
+
+router = APIRouter(prefix="/api/sync", tags=["sync"])
+
+
+@router.post("/subscriptions")
+def sync_subscriptions(
+    user: User = Depends(current_user), db: Session = Depends(get_db)
+) -> dict:
+    before = quota.units_used_today(db)
+    result = import_subscriptions(db, user)
+    result["quota_used_estimate"] = quota.units_used_today(db) - before
+    result["quota_remaining_today"] = quota.remaining_today(db)
+    return result
+
+
+@router.get("/status")
+def sync_status(
+    user: User = Depends(current_user), db: Session = Depends(get_db)
+) -> dict:
+    sub_count = db.scalar(
+        select(func.count())
+        .select_from(Subscription)
+        .where(Subscription.user_id == user.id)
+    )
+    return {
+        "subscriptions": sub_count,
+        "channels_total": db.scalar(select(func.count()).select_from(Channel)),
+        "videos_total": db.scalar(select(func.count()).select_from(Video)),
+        "quota_used_today": quota.units_used_today(db),
+        "quota_remaining_today": quota.remaining_today(db),
+    }