chore(dev): add pnpm dev hot-reload workflow for local development

Replace full docker compose rebuild during development with:
- pnpm dev / dev:backend (air hot-reload control-plane) / dev:frontend (vite HMR) / dev:deps
- compose.dev.yaml: postgres+docker-gateway only, host port mapping (5432/8081), container control-plane disabled via profile
- scripts/dev-backend.mjs: read .env, fill dev defaults (login admin/admin123, listen :8082, 32-byte dev master key with explicit warning on invalid .env value), ensure dep containers, run air
- vite: fixed port 5173, /api proxy to local control-plane (8082)
- README: document hot-reload dev workflow

Verified: pnpm dev end-to-end (login/API/frontend/proxy 200), Go+JSX edit hot-reload, compose config --quiet, go build ./..., web tests 65 passed, npm --prefix web run build.

compose.dev.yaml

@@ -0,0 +1,14 @@
+# 本地热加载开发用 override：只跑 postgres + docker-gateway，宿主机直接跑 control-plane 与前端。
+# 用法：docker compose -f compose.yaml -f compose.dev.yaml up -d postgres docker-gateway
+services:
+  creator-hub:
+    # 本地开发不跑容器版 control-plane；profile 化后不启动。
+    profiles: [compose-only]
+
+  postgres:
+    ports:
+      - "5432:5432"
+
+  docker-gateway:
+    ports:
+      - "8081:8081"