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.

Makefile

@@ -39,7 +39,7 @@ dev: dev-infra migrate-up dev-cp
 dev-infra:
 	docker compose -f deploy/docker-compose.dev.yml up -d
 	@echo "Waiting for PostgreSQL..."
-	@until pg_isready -h localhost -p 5432 -q; do sleep 0.5; done
+	@until docker compose -f deploy/docker-compose.dev.yml exec -T postgres pg_isready -q 2>/dev/null; do sleep 0.5; done
 	@echo "Dev infrastructure ready."
 
 dev-down:
@@ -53,7 +53,7 @@ dev-agent:
 	sudo go run ./cmd/host-agent
 
 dev-frontend:
-	cd frontend && pnpm dev --port 5173
+	cd frontend && pnpm dev --port 5173 --host 0.0.0.0
 
 dev-envd:
 	cd $(ENVD_DIR) && go run . --debug --listen-tcp :3002