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.

tests/setup/api.ts

@@ -127,7 +127,12 @@ vi.mock('@/lib/stripe', async (importOriginal) => {
     ...actual,
     getStripe: vi.fn(() => ({
       customers: { create: vi.fn(async () => ({ id: 'cus_test_default' })) },
-      subscriptions: { list: vi.fn(async () => ({ data: [] })) },
+      subscriptions: {
+        list: vi.fn(async () => ({ data: [] })),
+        update: vi.fn(() => {
+          throw new Error('stripe.subscriptions.update was not stubbed for this test');
+        }),
+      },
       checkout: {
         sessions: { create: vi.fn(async () => ({ url: 'https://stripe.test/checkout' })) },
       },