feat(stats): per-user API quota attribution + admin usage page

Track who burned how much YouTube API quota. A QuotaEvent audit log (migration
0009) records every spend with the triggering user (NULL = background/system) and
an action label, set via a request/job-scoped contextvar (quota.attribute) so no
call signatures change. User-initiated work (sync subscriptions, unsubscribe,
opt-in recent backfill, manual enrich) attributes to the user; scheduler work to
System, split by action.

- backend: QuotaEvent model + migration 0009; quota.attribute() contextvar;
  record_usage logs events; entry points wrapped (routes/sync, routes/channels,
  scheduler); GET /api/quota/my-usage + GET /api/quota/admin
- frontend: admin-only Stats page (header nav, page=stats) with daily bars +
  per-user breakdown by action and range picker; 'Your API usage' in Settings ->
  Sync for every user

Verified: attribution + endpoints compute correctly; events are per-user vs System.

backend/app/routes/sync.py

@@ -34,7 +34,8 @@ 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)
+    with quota.attribute(user.id, "sync_subscriptions"):
+        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
@@ -55,7 +56,8 @@ def sync_backfill(
     max_channels: int = 25,
 ) -> dict:
     before = quota.units_used_today(db)
-    result = run_recent_backfill(db, _user_channels(db, user), max_channels=max_channels)
+    with quota.attribute(user.id, "backfill_recent"):
+        result = run_recent_backfill(db, _user_channels(db, user), max_channels=max_channels)
     result["quota_used_estimate"] = quota.units_used_today(db) - before
     return result
 
@@ -65,7 +67,8 @@ def sync_enrich(
     user: User = Depends(current_user), db: Session = Depends(get_db)
 ) -> dict:
     before = quota.units_used_today(db)
-    enriched = run_enrich(db)
+    with quota.attribute(user.id, "enrich"):
+        enriched = run_enrich(db)
     return {
         "enriched": enriched,
         "quota_used_estimate": quota.units_used_today(db) - before,