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/backend/openapi/bookra.openapi.yaml

@@ -639,9 +639,16 @@ components:
         planCode:
           type: string
           enum: [starter, pro, business]
+          description: The plan to subscribe to
         currency:
           type: string
           enum: [czk, usd]
+          description: Currency for the subscription
+        billingInterval:
+          type: string
+          enum: [monthly, yearly]
+          default: monthly
+          description: Billing interval. Yearly gets 17% discount.
     PlanDisplayPrice:
       type: object
       required: [currency, amountCents, formatted]
@@ -651,29 +658,56 @@ components:
           enum: [czk, usd]
         amountCents:
           type: integer
+          description: Monthly price in cents
         formatted:
           type: string
+          description: Formatted monthly price string
+        yearlyAmountCents:
+          type: integer
+          description: Yearly price in cents (17% discount)
+        yearlyFormatted:
+          type: string
+          description: Formatted yearly price string
+        yearlySavings:
+          type: string
+          description: Description of yearly savings
+        yearlySavingsPercent:
+          type: integer
+          description: Percentage saved with yearly billing
     CheckoutLaunchResponse:
       type: object
-      required: [priceId, successRedirectUrl, cancelRedirectUrl, customData]
+      description: |
+        Checkout launch response supporting both Stripe and Paddle providers.
+        For Stripe: checkoutUrl is returned (redirect-based checkout).
+        For Paddle: priceId, customerId, customerEmail, customData are returned (client-side checkout).
       properties:
+        checkoutUrl:
+          type: string
+          format: uri
+          description: Stripe checkout URL (redirect the user to this URL)
         priceId:
           type: string
+          description: Paddle price ID for client-side checkout
         customerId:
           type: string
+          description: Paddle customer ID
         customerEmail:
           type: string
           format: email
+          description: Customer email for Paddle checkout
         successRedirectUrl:
           type: string
           format: uri
+          description: URL to redirect after successful checkout
         cancelRedirectUrl:
           type: string
           format: uri
+          description: URL to redirect after cancelled checkout
         customData:
           type: object
           additionalProperties:
             type: string
+          description: Custom metadata for Paddle checkout
     PortalSessionResponse:
       type: object
       required: [url]