feat: add annual billing, free trial, upgrade/downgrade, and ACH invoice support

- Add monthly/annual billing toggle with 25% annual discount on pricing page
- Implement 14-day no-card free trial (server-side Stripe subscription creation)
- Enable upgrade/downgrade via Stripe Customer Portal
- Add admin-initiated ACH/invoice billing for enterprise customers
- Add billing card to Settings page with plan info and Manage Billing button
- Handle past_due status with read-only grace period access
- Add trial ending and trial expired email templates
- Add DB migration for billing_interval and collection_method columns
- Update ONBOARDING-AND-AUTH.md documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/common/guards/write-access.guard.ts

@@ -30,6 +30,13 @@ export class WriteAccessGuard implements CanActivate {
       throw new ForbiddenException('Read-only users cannot modify data');
     }
 
+    // Block writes for past_due organizations (grace period: read-only access)
+    if (request.orgPastDue) {
+      throw new ForbiddenException(
+        'Your subscription is past due. Please update your payment method to continue making changes.',
+      );
+    }
+
     return true;
   }
 }