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/database/tenant.middleware.ts

@@ -9,6 +9,7 @@ export interface TenantRequest extends Request {
   orgId?: string;
   userId?: string;
   userRole?: string;
+  orgPastDue?: boolean;
 }
 
 @Injectable()
@@ -41,6 +42,10 @@ export class TenantMiddleware implements NestMiddleware {
               });
               return;
             }
+            // past_due: allow through with read-only flag (WriteAccessGuard enforces)
+            if (orgInfo.status === 'past_due') {
+              req.orgPastDue = true;
+            }
             req.tenantSchema = orgInfo.schemaName;
           }
           req.orgId = decoded.orgId;