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/)

frontend/nginx.conf

@@ -1,25 +1,24 @@
 server {
-    listen 80;
-    server_name localhost;
+  listen 80;
+  root /usr/share/nginx/html;
+  index index.html;
 
-    root /usr/share/nginx/html;
-    index index.html;
+  resolver 127.0.0.11 valid=30s;
 
-    # Serve static frontend
-    location / {
-        try_files $uri /index.html;
-    }
+  location / { try_files $uri $uri/ /index.html; }
 
-    # Proxy API requests to backend
-    location /api/ {
-        proxy_pass http://backend:5000;
-        proxy_http_version 1.1;
-        proxy_set_header Upgrade $http_upgrade;
-        proxy_set_header Connection 'upgrade';
-        proxy_set_header Host $host;
-        proxy_cache_bypass $http_upgrade;
-        proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Forwarded-Proto $scheme;
-    }
+  location /api/ {
+    set $backend http://backend:3001;
+    proxy_pass $backend;
+    proxy_set_header Host $host;
+    proxy_set_header X-Real-IP $remote_addr;
+  }
+
+  location /ws {
+    set $backend http://backend:3001;
+    proxy_pass $backend;
+    proxy_http_version 1.1;
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection "upgrade";
+  }
 }