feat(core): consolidate auth service into backend and implement stripe billing

This commit performs a major architectural refactor by migrating the standalone `auth-service` into the main `backend` application, enabling a unified codebase and simplified deployment. It also introduces comprehensive Stripe billing support and a new administrative dashboard.

Key changes:
- **Architecture**: Deleted `apps/auth-service` and integrated its functionality (JWT, magic links, OAuth, user management) into `apps/backend`.
- **Billing**: Added Stripe integration to `backend`, supporting both monthly and yearly subscription cycles with automatic plan entitlement enforcement (e.g., location limits).
- **Admin Dashboard**: Implemented a new administrative service and API endpoints to manage tenants, users, and view platform-wide statistics.
- **Frontend**:
    - Added a new pricing page with monthly/yearly toggle and comparison table.
    - Integrated Stripe and Sentry for payments and error tracking.
    - Improved dashboard UX/UI and added i18n support for new features.
    - Enhanced the public booking flow with better validation and contact form integration.
- **Database**: Added migrations for users, magic links, password resets, OAuth states, admin audit logs, and refresh tokens.
- **DevOps**: Updated environment configurations for Railway and Vercel, and streamlined the project's `package.json` scripts.

apps/frontend/src/routes/booking-manage-route.tsx

@@ -40,6 +40,7 @@ export function BookingManageRoute() {
       customerEmail: "[email protected]",
       service: "Yoga Flow Class",
       businessName: "Serenity Wellness Studio",
+      businessEmail: "[email protected]",
       startsAt: new Date(Date.now() + 86400000).toISOString(),
       endsAt: new Date(Date.now() + 86400000 + 3600000).toISOString(),
       location: "Main Studio, 123 Wellness Street",
@@ -331,7 +332,7 @@ export function BookingManageRoute() {
                         : 'Have questions or need special arrangements? Contact the business directly.'}
                     </p>
                     <a 
-                      href={`mailto:[email protected]?subject=Booking ${b().reference}`}
+                      href={`mailto:${b().businessEmail || '[email protected]'}?subject=Booking ${b().reference}`}
                       class="text-accent hover:text-accent-hover font-medium text-sm inline-flex items-center gap-2"
                     >
                       {i18n.locale() === 'cs' ? 'Poslat zprávu' : 'Send message'}