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/App.tsx

@@ -22,6 +22,7 @@ import Header from "./components/Header";
 import Sidebar from "./components/Sidebar";
 import Feed from "./components/Feed";
 import Channels from "./components/Channels";
+import Stats from "./components/Stats";
 import SettingsPanel from "./components/SettingsPanel";
 import Toaster from "./components/Toaster";
 
@@ -157,9 +158,7 @@ export default function App() {
           />
         )}
         <main className="flex-1 min-w-0 overflow-y-auto">
-          {page === "feed" ? (
-            <Feed filters={filters} setFilters={setFilters} view={view} />
-          ) : (
+          {page === "channels" ? (
             <Channels
               canWrite={meQuery.data!.can_write}
               onViewChannel={(id, name) => {
@@ -167,6 +166,10 @@ export default function App() {
                 setPage("feed");
               }}
             />
+          ) : page === "stats" && meQuery.data!.role === "admin" ? (
+            <Stats />
+          ) : (
+            <Feed filters={filters} setFilters={setFilters} view={view} />
           )}
         </main>
       </div>