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

sdk/plugin_web.py

@@ -0,0 +1,50 @@
+"""PluginWebMixin — plugin web panel SDK"""
+import logging, json, asyncio
+from aiohttp import web
+from typing import Dict, Callable
+
+logger = logging.getLogger(__name__)
+
+class PluginWebMixin:
+    def __init__(self):
+        self._web_pages: Dict[str, dict] = {}
+        self._api_routes: list = []
+        self._sse_clients: list = []
+
+    def register_web_page(self, path: str, title: str, html_content: str, icon: str = "P"):
+        self._web_pages[path] = {"title": title, "icon": icon, "html": html_content}
+        logger.info(f"Web page registered: {path}")
+
+    def register_api_route(self, method: str, path: str, handler: Callable):
+        self._api_routes.append((method, path, handler))
+
+    def get_web_pages(self) -> dict:
+        return self._web_pages
+
+    def get_api_routes(self) -> list:
+        return self._api_routes
+
+    async def push_sse_event(self, event_type: str, data: dict):
+        payload = json.dumps(data)
+        dead = []
+        for client in self._sse_clients:
+            try:
+                await client.send(f"event: {event_type}\\ndata: {payload}\\n\\n")
+            except:
+                dead.append(client)
+        for d in dead:
+            self._sse_clients.remove(d)
+
+    async def handle_sse(self, request):
+        resp = web.StreamResponse()
+        resp.headers["Content-Type"] = "text/event-stream"
+        resp.headers["Cache-Control"] = "no-cache"
+        await resp.prepare(request)
+        self._sse_clients.append(resp)
+        try:
+            while True:
+                await asyncio.sleep(30)
+                await resp.write(b": keepalive\\n\\n")
+        except:
+            self._sse_clients.remove(resp)
+        return resp