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.

.env.example

@@ -0,0 +1,10 @@
+# Postgres connection used by Prisma
+DATABASE_URL="postgresql://postgres:postgres@localhost:5432/kcapp?schema=public"
+
+# Authentik OIDC issuer, e.g. https://auth.example.org/application/o/kc-app/
+AUTHENTIK_ISSUER_URL="https://authentik.example.org/application/o/kc-app"
+
+# Secret used to sign guest/Konfi session tokens (local accounts only)
+GUEST_JWT_SECRET="change-me"
+
+PORT=3000