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/SettingsPanel.tsx

@@ -3,7 +3,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Bell, Check, History, Monitor, Pause, Play, RefreshCw, User, UserPlus, X } from "lucide-react";
 import { SCHEMES, type Scheme, type ThemePrefs } from "../lib/theme";
 import { api, type Invite, type Me } from "../lib/api";
-import { formatEta } from "../lib/format";
+import { formatEta, quotaActionLabel } from "../lib/format";
 import {
   configureNotifications,
   getNotifSettings,
@@ -322,6 +322,7 @@ function Notifications() {
 function Sync({ me }: { me: Me }) {
   const qc = useQueryClient();
   const status = useQuery({ queryKey: ["my-status"], queryFn: api.myStatus });
+  const usage = useQuery({ queryKey: ["my-usage"], queryFn: api.myUsage });
   const s = status.data;
   const syncSubs = useMutation({
     mutationFn: () => api.syncSubscriptions(),
@@ -392,6 +393,36 @@ function Sync({ me }: { me: Me }) {
         )}
       </Section>
 
+      <Section title="Your API usage">
+        {usage.data ? (
+          <>
+            <div className="space-y-1.5 text-sm">
+              <Row label="Today" hint="YouTube API units your own actions used today (resets midnight US Pacific). Background sync isn't counted here.">
+                {usage.data.today.toLocaleString()}
+              </Row>
+              <Row label="Last 7 days">{usage.data.last_7d.toLocaleString()}</Row>
+              <Row label="Last 30 days">{usage.data.last_30d.toLocaleString()}</Row>
+              <Row label="All time">{usage.data.all_time.toLocaleString()}</Row>
+            </div>
+            {Object.keys(usage.data.by_action).length > 0 && (
+              <div className="mt-2 pt-2 border-t border-border space-y-1 text-xs text-muted">
+                <div className="uppercase tracking-wide">By action (30d)</div>
+                {Object.entries(usage.data.by_action)
+                  .sort((a, b) => b[1] - a[1])
+                  .map(([action, units]) => (
+                    <div key={action} className="flex justify-between gap-2">
+                      <span>{quotaActionLabel(action)}</span>
+                      <span className="tabular-nums">{units.toLocaleString()}</span>
+                    </div>
+                  ))}
+              </div>
+            )}
+          </>
+        ) : (
+          <div className="text-muted text-sm">No usage yet.</div>
+        )}
+      </Section>
+
       <Section title="Actions">
         <Tooltip hint="Re-import your YouTube subscriptions and pull in any newly-followed channels.">
           <button