feat(admin): add full-chain trace and performance monitoring

- Backend: trace.Logger ring-buffer for request chain events
- Backend: perf.Monitor with runtime.MemStats sampling every 10s
- Backend: /api/v1/admin/trace and /api/v1/admin/perf endpoints
- Frontend: TracePage — timeline with hop icons and status dots
- Frontend: PerformancePage — memory/GC bar charts + live stats
- Admin nav expanded to 11 items

backend/services/gateway/api/router.go

@@ -66,6 +66,17 @@ type LLMCallLogger interface {
 	Stats() map[string]any
 }
 
+// TraceLogger provides full-chain trace events.
+type TraceLogger interface {
+	Recent(n int) []map[string]any
+}
+
+// PerfMonitor provides performance snapshot history.
+type PerfMonitor interface {
+	Latest() map[string]any
+	History() []map[string]any
+}
+
 // Dependencies holds all injectable services.
 type Dependencies struct {
 	AuthMgr      AuthManager
@@ -74,6 +85,8 @@ type Dependencies struct {
 	Orchestrator OrchestratorPipeline
 	WSHub        WebSocketHub
 	LLMLogger    LLMCallLogger
+	TraceLog     TraceLogger
+	PerfMon      PerfMonitor
 	ModelConfig  *ModelConfigStore
 	PluginList   []map[string]string
 }
@@ -118,6 +131,8 @@ func (r *Router) setupRoutes() {
 	r.mux.HandleFunc("/api/v1/admin/sessions", r.authMW(r.adminMW(r.adminSessionsHandler)))
 	r.mux.HandleFunc("/api/v1/admin/models/config", r.authMW(r.adminMW(r.modelConfigHandler)))
 	r.mux.HandleFunc("/api/v1/admin/models/config/", r.authMW(r.adminMW(r.modelConfigItemHandler)))
+	r.mux.HandleFunc("/api/v1/admin/trace", r.authMW(r.adminMW(r.traceHandler)))
+	r.mux.HandleFunc("/api/v1/admin/perf", r.authMW(r.adminMW(r.perfHandler)))
 }
 
 // ServeHTTP implements http.Handler.
@@ -486,6 +501,27 @@ func num(m map[string]any, key string) int {
 	return 0
 }
 
+// --- Trace & Perf ---
+
+func (r *Router) traceHandler(w http.ResponseWriter, req *http.Request) {
+	if r.deps.TraceLog == nil {
+		writeJSON(w, 200, M{"events": []any{}})
+		return
+	}
+	writeJSON(w, 200, M{"events": r.deps.TraceLog.Recent(100)})
+}
+
+func (r *Router) perfHandler(w http.ResponseWriter, req *http.Request) {
+	if r.deps.PerfMon == nil {
+		writeJSON(w, 200, M{"latest": M{}, "history": []any{}})
+		return
+	}
+	writeJSON(w, 200, M{
+		"latest":  r.deps.PerfMon.Latest(),
+		"history": r.deps.PerfMon.History(),
+	})
+}
+
 // --- Helpers ---
 
 type M = map[string]any