WIP: Add Caddy reverse proxy for dev environment

Add Caddy to docker-compose as the single entry point on port 8000:
- localhost -> /api/* stripped and proxied to CP:8080, /* to frontend:5173
- *.localhost -> proxied to CP:8080 (sandbox proxy catch-all)
- Direct /v1/*, /auth/*, /docs routes proxied to CP

Move CP from :8000 to :8080 (its default). Caddy takes :8000.
Update .env.example, vite proxy target (kept as fallback), and Makefile
dev targets (pg_isready via docker exec, frontend binds 0.0.0.0).

This is an intermediate state — needs further work for the full code
interpreter feature.

frontend/vite.config.ts

@@ -7,7 +7,7 @@ export default defineConfig({
 	server: {
 		proxy: {
 			'/api': {
-				target: 'http://localhost:8000',
+				target: 'http://localhost:8080',
 				rewrite: (path) => path.replace(/^\/api/, '')
 			}
 		}