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.

src/auth/authenticated-request.ts

@@ -0,0 +1,20 @@
+import { Request } from 'express';
+import { Role } from '../common/role.enum';
+
+export interface AuthenticatedMembership {
+  kcId: string;
+  gemeindeId: string | null;
+  role: Role;
+}
+
+/// Shape attached to req.user by JwtStrategy after validating an access token.
+export interface AuthenticatedUser {
+  userId: string;
+  authentikSub: string;
+  email: string;
+  memberships: AuthenticatedMembership[];
+}
+
+export interface AuthenticatedRequest extends Request {
+  user?: AuthenticatedUser;
+}