feat(admin): add model config CRUD and platform bridge pages

- Backend: ModelConfigStore with Provider/Model/Routing CRUD
- Backend: /api/v1/admin/models/config endpoints
- Frontend: ModelsPage with 3-tab editor (providers/models/routing)
- Frontend: BridgePage showing adapter status + group chat rules
- Admin nav expanded to 9 items

backend/cmd/cyrene/main.go

@@ -237,6 +237,7 @@ func main() {
 		AuthMgr:      authMgr,
 		Orchestrator: orch,
 		LLMLogger:    &llmLoggerAdapter{Logger: llmLogger},
+		ModelConfig:  api.NewModelConfigStore(),
 		PluginList:   pluginNamesToMap(reg.ListNames()),
 	}
 	if memSvc != nil {