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/components/Header.tsx

@@ -1,5 +1,5 @@
 import { useRef, useState } from "react";
-import { Home, LogOut, Search, Settings, Shield, Tv } from "lucide-react";
+import { BarChart3, Home, LogOut, Search, Settings, Shield, Tv } from "lucide-react";
 import type { FeedFilters, Me } from "../lib/api";
 import type { Page } from "../lib/urlState";
 import SyncStatus from "./SyncStatus";
@@ -48,7 +48,9 @@ export default function Header({
           />
         </div>
       ) : (
-        <div className="flex-1 text-center text-sm font-semibold">Channel manager</div>
+        <div className="flex-1 text-center text-sm font-semibold">
+          {page === "stats" ? "Usage & stats" : "Channel manager"}
+        </div>
       )}
 
       <div className="flex items-center gap-1">
@@ -134,17 +136,24 @@ function AccountMenu({
           )}
 
           <div className="mt-2 flex flex-col">
-            {page === "channels" ? (
+            {page !== "feed" && (
               <button onClick={() => { setPage("feed"); setOpen(false); }} className={itemClass}>
                 <Home className="w-4 h-4" />
                 Feed
               </button>
-            ) : (
+            )}
+            {page !== "channels" && (
               <button onClick={() => { setPage("channels"); setOpen(false); }} className={itemClass}>
                 <Tv className="w-4 h-4" />
                 Channels
               </button>
             )}
+            {me.role === "admin" && page !== "stats" && (
+              <button onClick={() => { setPage("stats"); setOpen(false); }} className={itemClass}>
+                <BarChart3 className="w-4 h-4" />
+                Stats
+              </button>
+            )}
             <button onClick={() => { onOpenSettings(); setOpen(false); }} className={itemClass}>
               <Settings className="w-4 h-4" />
               Settings