feat: initialize backend with NestJS, PostgreSQL, and Prisma

- Add package.json for backend dependencies and scripts.
- Create Prisma schema for multi-tenant event management.
- Implement main application module and configure global settings.
- Develop authentication module with JWT and Authentik integration.
- Create DTOs for guest account creation and KC management.
- Implement role-based access control with custom guards and decorators.
- Add services and controllers for managing KCs and guest accounts.
- Set up global validation and CORS in the main application entry point.
- Establish Prisma module for database access throughout the application.
- Document project plan and architecture for multi-tenant platform.

backend/src/common/role.enum.ts

@@ -0,0 +1,4 @@
+/// Re-exported from the Prisma client so guards and strategies share one
+/// enum type with the database schema. Guests are not part of this enum
+/// since they authenticate separately and never hold elevated rights.
+export { Role } from '@prisma/client';