feat(billing): cancel in-app with a one-question reason

Add a "Cancel subscription" button beside "Manage Subscription" in Settings.
It opens a dialog with one optional question (five answers, no default, a
note box under the two that want detail), then schedules the Stripe
subscription to end at the close of the current period without a trip to
the portal. The answer is stored in a new subscription_cancellations table
and shown, with an all-time tally, on the admin dashboard; the category is
also mirrored onto Stripe's cancellation feedback, the free text stays local.

The cancel route claims the local cancel flag with a conditional update
before calling Stripe, so two racing requests cannot both write a reason
row, and hands the claim back when Stripe refuses. A subscription Stripe no
longer knows answers 409 with a pointer to the portal instead of a 500. The
route carries an account-keyed rate limit on top of the shared IP one.

Two fixes found on the way: the pinned Stripe API version reports
current_period_end on the subscription item rather than the subscription, so
the sync stored null for every period end; a shared helper now reads the item
first. And the RadioGroup styles targeted a data-checked attribute radix
never writes, so the checked state was invisible in the light theme.

app/admin/page.tsx

@@ -1,4 +1,5 @@
 import { Metadata } from 'next';
+import { Suspense } from 'react';
 import { db } from '@/lib/db';
 import { auth } from '@/lib/auth';
 import { isBunnyUploadsFeatureEnabled, isStripeBillingEnabled } from '@/lib/feature-flags';
@@ -10,6 +11,7 @@ import {
 } from '@/lib/admin-stats';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { RefreshR2StatsButton } from '@/components/admin/refresh-r2-stats-button';
+import { CancellationReasonsCard } from '@/components/admin/cancellation-reasons-card';
 import {
   Users,
   Folder,
@@ -289,6 +291,14 @@ export default async function AdminDashboardPage() {
           </div>
         </>
       )}
+
+      {/* Outside the `stripeStats` guard on purpose: the answers live in our own
+          table and must stay readable while a Stripe outage blanks the cards above. */}
+      {isStripeBillingEnabled() && (
+        <Suspense fallback={null}>
+          <CancellationReasonsCard />
+        </Suspense>
+      )}
     </div>
   );
 }