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/api.ts

@@ -165,6 +165,27 @@ export interface MyStatus {
   paused: boolean;
 }
 
+export interface MyUsage {
+  today: number;
+  last_7d: number;
+  last_30d: number;
+  all_time: number;
+  by_action: Record<string, number>;
+}
+
+export interface AdminQuotaRow {
+  user_id: number | null;
+  email: string;
+  total: number;
+  by_action: Record<string, number>;
+}
+
+export interface AdminQuota {
+  range_days: number;
+  rows: AdminQuotaRow[];
+  daily: { day: string; total: number }[];
+}
+
 export interface ManagedChannel {
   id: string;
   title: string | null;
@@ -215,6 +236,10 @@ export const api = {
     req(`/api/channels/${id}/subscription`, { method: "DELETE" }),
   syncSubscriptions: () => req("/api/sync/subscriptions", { method: "POST" }),
 
+  // --- quota usage ---
+  myUsage: (): Promise<MyUsage> => req("/api/quota/my-usage"),
+  adminQuota: (days = 30): Promise<AdminQuota> => req(`/api/quota/admin?days=${days}`),
+
   // --- onboarding / admin ---
   requestAccess: (email: string): Promise<{ status: string }> =>
     req("/auth/request-access", { method: "POST", body: JSON.stringify({ email }) }),

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";

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. */