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/auth/guest-auth.service.ts

@@ -0,0 +1,41 @@
+import { Injectable, NotFoundException } from '@nestjs/common';
+import { JwtService } from '@nestjs/jwt';
+import { PrismaClient } from '../prisma/prisma.module';
+
+export interface GuestJwtPayload {
+  guestId: string;
+  kcId: string;
+  gemeindeId: string | null;
+}
+
+/// Guest/Konfi accounts are local to this server (never Authentik-backed),
+/// created via a KC invite code, and scoped to that single KC.
+@Injectable()
+export class GuestAuthService {
+  constructor(
+    private readonly prisma: PrismaClient,
+    private readonly jwt: JwtService,
+  ) {}
+
+  async createGuest(
+    inviteCode: string,
+    firstName: string,
+    lastName: string,
+  ): Promise<{ accessToken: string }> {
+    const kc = await this.prisma.kc.findUnique({ where: { inviteCode } });
+    if (!kc || !kc.isActive) {
+      throw new NotFoundException('Unknown or inactive KC invite code');
+    }
+
+    const guest = await this.prisma.guestAccount.create({
+      data: { kcId: kc.id, firstName, lastName },
+    });
+
+    const payload: GuestJwtPayload = {
+      guestId: guest.id,
+      kcId: kc.id,
+      gemeindeId: guest.gemeindeId,
+    };
+    return { accessToken: await this.jwt.signAsync(payload) };
+  }
+}