Clean working version: deployed TypeScript backend + JSX frontend

- Replace old JS backend with working TypeScript backend from deployed containers
- Backend: Fastify + better-sqlite3 + Yjs CRDT, 17 TS source files, 13 test files
- Frontend: React JSX with fabric.js CAD canvas, vite build, nginx serving
- Fix nginx proxy port 5000→3001 to match backend
- Fix docker-compose port mapping 5000→3001
- Fix vite dev proxy port 5000→3001
- Add backend healthcheck to docker-compose
- Update index.html title to 'Web CAD', lang to 'de'
- Add .a0/ to .gitignore
- Remove old JS backend files (models, routes, config, middleware, seed)
- Remove tracked build artifacts (backend/public/)

docker-compose.yml

@@ -4,12 +4,17 @@ services:
   backend:
     build: ./backend
     ports:
-      - "5000:5000"
+      - "3001:3001"
     environment:
       - NODE_ENV=production
       - JWT_SECRET=your-secret-key-change-me
     volumes:
       - sqlite_data:/app/data
+    healthcheck:
+      test: ["CMD", "wget", "--spider", "-q", "http://localhost:3001/api/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
     restart: unless-stopped
 
   frontend: