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.

frontend/src/lib/format.ts

@@ -34,6 +34,20 @@ export function formatDuration(sec: number | null): string {
   return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
 }
 
+const QUOTA_ACTION_LABELS: Record<string, string> = {
+  sync_subscriptions: "Sync subscriptions",
+  backfill_recent: "Recent backfill",
+  backfill_deep: "Full-history backfill",
+  enrich: "Enrichment",
+  unsubscribe: "Unsubscribe",
+  subscription_resync: "Auto subscription resync",
+  api: "Other",
+};
+
+export function quotaActionLabel(action: string): string {
+  return QUOTA_ACTION_LABELS[action] ?? action;
+}
+
 /** Coarse, human ETA for a remaining duration in seconds ("~3 hours", "~2 days"). */
 export function formatEta(seconds: number): string {
   if (seconds <= 0) return "done";