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