Phase 3: PluginWebMixin SDK + web plugin control panels

- New: sdk/plugin_web.py (PluginWebMixin: web pages, API routes, SSE)
- New: services/web_panel/routes/plugin_web.py (3 endpoints)
- New: plugins/example_plugin/dashboard.html (counter + SSE demo)
- Updated: example_plugin uses PluginWebMixin
- Tests: 28/28 passing

docs/Phase3_Progress.md

@@ -0,0 +1,20 @@
+# Phase 3 开发进度总结
+
+> 完成: 2026-06-10
+> 测试: 28/28 通过
+
+## 新增
+- `sdk/plugin_web.py` — PluginWebMixin (Web页面注册+API+SSE)
+- `services/web_panel/routes/plugin_web.py` — 插件面板路由
+- `plugins/example_plugin/dashboard.html` — 示例仪表盘 (计数器+SSE)
+
+## SDK API
+- `register_web_page(path, title, html, icon)` — 注册控制页面
+- `register_api_route(method, path, handler)` — 注册 REST 端点
+- `push_sse_event(type, data)` — SSE 实时推送
+- `handle_sse(request)` — SSE 连接处理
+
+## 端点
+- GET /SenSu/plugin/{name} — 插件控制面板
+- GET /SenSu/plugin/{name}/sse — SSE 事件流
+- POST /SenSu/plugin/{name}/event — 触发事件