feat(billing): defer the cardless trial for invited collaborators

An account that signs up through an invitation works on the inviter's
billing, so handing it a trial at signup spent its only trial before it
owned anything. The trial is now held back for collaborators and claimed
only explicitly: a Start Free Trial button on the new-workspace and
billing screens calls the new POST /api/billing/trial endpoint, which
grants the once-per-account trial atomically. Nothing starts the clock
as a side effect, and pure collaborators no longer see a trial-ending
banner about work that is not theirs.

app/api/auth/register/route.ts

@@ -22,7 +22,7 @@ import {
   isValidEmailAddress,
   normalizeEmail,
 } from '@/lib/email-validation';
-import { startCardlessTrial } from '@/lib/billing';
+import { startCardlessTrialOnSignup } from '@/lib/billing';
 import { recordSignupCompleted } from '@/lib/analytics/signup';
 import { readRequestVisitor } from '@/lib/analytics/visitor';
 
@@ -166,7 +166,7 @@ export async function POST(request: NextRequest) {
     // just lock the user out of an instance that has billing switched on.
     if (!emailVerificationRequired) {
       warnIfTrialsSkipVerification();
-      await startCardlessTrial(user.id);
+      await startCardlessTrialOnSignup(user.id);
     }
 
     // Send verification email if SMTP is configured

app/api/billing/trial/route.ts

@@ -0,0 +1,52 @@
+import { NextRequest } from 'next/server';
+import { auth } from '@/lib/auth';
+import { apiErrors, successResponse } from '@/lib/api-response';
+import { startCardlessTrial } from '@/lib/billing';
+import { rateLimit } from '@/lib/rate-limit';
+import { isStripeFeatureEnabled } from '@/lib/feature-flags';
+import { isTrustedSameOriginRequest } from '@/lib/request-origin';
+import { logError } from '@/lib/logger';
+import { db } from '@/lib/db';
+
+/**
+ * The explicit claim of a deferred cardless trial.
+ *
+ * An invited collaborator has their trial held back at signup; nothing else in
+ * the product is allowed to start it as a side effect, because the clock spends
+ * the account's only trial. This endpoint is the one place the user says "start
+ * it now", from the workspace-creation and billing screens.
+ */
+export async function POST(request: NextRequest) {
+  try {
+    const limited = await rateLimit(request, 'mutate');
+    if (limited) return limited;
+
+    if (!isTrustedSameOriginRequest(request)) {
+      return apiErrors.forbidden('Invalid request origin');
+    }
+
+    const session = await auth();
+    if (!session?.user?.id) {
+      return apiErrors.unauthorized();
+    }
+
+    if (!isStripeFeatureEnabled()) {
+      return apiErrors.badRequest('Stripe billing is disabled by this host');
+    }
+
+    const started = await startCardlessTrial(session.user.id);
+    if (!started) {
+      return apiErrors.conflict('Your free trial has already been used');
+    }
+
+    const user = await db.user.findUnique({
+      where: { id: session.user.id },
+      select: { trialEndsAt: true },
+    });
+
+    return successResponse({ trialEndsAt: user?.trialEndsAt ?? null });
+  } catch (error) {
+    logError('billing.trial.start', error);
+    return apiErrors.internalError();
+  }
+}