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

services/web_panel/routes/plugin_web.py

@@ -0,0 +1,38 @@
+from aiohttp import web
+import json, logging
+logger = logging.getLogger(__name__)
+
+def setup_plugin_web_routes(app, service_manager):
+    async def plugin_page(request):
+        name = request.match_info.get("name","")
+        ps = service_manager.get_service("plugin")
+        plugin = ps.plugins.get(name)
+        if not plugin or not hasattr(plugin, "get_web_pages"):
+            return web.Response(text=f"Plugin {name} not found", status=404)
+        pages = plugin.get_web_pages()
+        if name in pages:
+            return web.Response(text=pages[name]["html"], content_type="text/html")
+        return web.json_response({"error": "no web page"})
+
+    async def plugin_sse(request):
+        name = request.match_info.get("name","")
+        ps = service_manager.get_service("plugin")
+        plugin = ps.plugins.get(name)
+        if plugin and hasattr(plugin, "handle_sse"):
+            return await plugin.handle_sse(request)
+        return web.json_response({"error": "SSE not supported"}, status=404)
+
+    async def plugin_event(request):
+        name = request.match_info.get("name","")
+        data = await request.json()
+        ps = service_manager.get_service("plugin")
+        plugin = ps.plugins.get(name)
+        if plugin and hasattr(plugin, "push_sse_event"):
+            await plugin.push_sse_event(data.get("type","event"), data)
+            return web.json_response({"ok": True})
+        return web.json_response({"ok": False}, status=404)
+
+    app.router.add_get("/plugin/{name}", plugin_page)
+    app.router.add_get("/plugin/{name}/sse", plugin_sse)
+    app.router.add_post("/plugin/{name}/event", plugin_event)
+    logger.info("Plugin web routes registered")