Fix build issues found during Docker validation

- Dockerfile: add COPY db/ /app/db/ so migrations are available at
  runtime (server/src/db.js requires ../../db/migrations at startup)
- vite.config.js: fix dev proxy port from 3001 to 3000 to match
  server's default PORT

Docker build passes; server starts and connects to DB correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/vite.config.js

@@ -5,7 +5,7 @@ export default defineConfig({
   plugins: [react()],
   server: {
     proxy: {
-      '/api': 'http://localhost:3001',
+      '/api': 'http://localhost:3000',
     },
   },
 });