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/urlState.ts

@@ -74,12 +74,11 @@ export function hasFilterParams(params: URLSearchParams): boolean {
   return KEYS.some((k) => params.has(k));
 }
 
-export type Page = "feed" | "channels";
+export type Page = "feed" | "channels" | "stats";
 
 export function readPage(): Page {
-  return new URLSearchParams(window.location.search).get("page") === "channels"
-    ? "channels"
-    : "feed";
+  const p = new URLSearchParams(window.location.search).get("page");
+  return p === "channels" || p === "stats" ? p : "feed";
 }
 
 /** Reflect the current filters + page into the address bar without adding history entries. */