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.

README.md

@@ -1,2 +1,26 @@
 # KC-APP
 
+Multi-tenant event, election and communication platform for Konfi-Castle
+events (KCs), replacing the WordPress plugin "Workshop-Wahlen". See
+[plan-kcAppMultiTenantPlatform.prompt.md](plan-kcAppMultiTenantPlatform.prompt.md)
+for the full architecture and phased roadmap.
+
+## Structure
+
+- `backend/` — NestJS API (Prisma/PostgreSQL, Authentik OIDC as resource
+  server, guest/Konfi local accounts, roles/permissions foundation). See
+  [backend/README.md](backend/README.md) for setup.
+- `client/` — planned Flutter app (mobile + web + desktop), not yet
+  scaffolded (Flutter is not installed in this environment).
+
+## Status
+
+Phase 0/1 foundation implemented: monorepo skeleton, Prisma data model (Kc,
+Gemeinde, User, Membership, GuestAccount, Wahl/Workshop/Teilnehmer/Zuteilung,
+File, Chat), Authentik JWT resource-server strategy, guest invite-code login,
+Role-based guard scoped per KC. Backend builds and boots cleanly
+(`npm run build`, `node dist/main.js`) but requires a real PostgreSQL
+database and Authentik instance (see `backend/.env.example`) to run end to
+end. Remaining phases (Wahl-Engine, Dateifreigabe, Chat realtime, Lokal/Cloud
+Sync, Flutter clients) are not yet implemented.
+