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.

prisma/migrations/20260908120000_add_subscription_cancellations/migration.sql

@@ -0,0 +1,22 @@
+-- One row per in-app cancellation, carrying the single answer the customer
+-- gave on the way out. Written when the request is made, before Stripe
+-- confirms it, so a reason is never lost to a webhook that arrives late.
+CREATE TYPE "CancellationReason" AS ENUM ('NOT_USING', 'MISSING_FEATURE', 'PRICE_OR_BILLING', 'PROJECT_ENDED', 'OTHER');
+
+CREATE TABLE "subscription_cancellations" (
+    "id" TEXT NOT NULL,
+    "userId" TEXT NOT NULL,
+    "stripeSubscriptionId" TEXT NOT NULL,
+    "reason" "CancellationReason",
+    "note" VARCHAR(500),
+    "periodEnd" TIMESTAMP(3),
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "subscription_cancellations_pkey" PRIMARY KEY ("id")
+);
+
+CREATE INDEX "subscription_cancellations_userId_createdAt_idx" ON "subscription_cancellations"("userId", "createdAt" DESC);
+CREATE INDEX "subscription_cancellations_createdAt_idx" ON "subscription_cancellations"("createdAt" DESC);
+
+ALTER TABLE "subscription_cancellations" ADD CONSTRAINT "subscription_cancellations_userId_fkey"
+    FOREIGN KEY ("userId") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;