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/)

backend/src/routes/settings.ts

@@ -0,0 +1,26 @@
+/**
+ * Settings Routes – key/value application settings
+ */
+import type { FastifyInstance } from 'fastify';
+import type { DatabaseInterface } from '../database/DatabaseInterface.js';
+
+export function registerSettingsRoutes(fastify: FastifyInstance, db: DatabaseInterface) {
+  // Get a single setting by key
+  fastify.get('/api/settings/:key', async (request, reply) => {
+    const { key } = request.params as { key: string };
+    const setting = db.getSetting(key);
+    if (!setting) return reply.code(404).send({ error: 'Setting not found' });
+    return setting;
+  });
+
+  // Create or update a setting (upsert)
+  fastify.put('/api/settings/:key', async (request) => {
+    const { key } = request.params as { key: string };
+    const body = request.body as { value?: string };
+    if (body.value === undefined) {
+      return { error: 'Value is required' };
+    }
+    db.setSetting(key, body.value);
+    return { key, value: body.value, updated_at: new Date().toISOString() };
+  });
+}