feat(observability): 管理面板可观测性接真实数据——metrics/trace/audit 三端点 + 审计持久化

- observability.go/observability_metrics.go/observability_trace.go/observability_audit.go：
  拆分保证 ≤500行/函数≤80。metrics 并发安全 MetricsCollector(方法/路径/状态计数、平均/最大延迟、
  近1s QPS窗口)；trace 并发安全 RingBuffer(默认512, newest-first)+traceSink+MultiTraceWriter；
  audit AuditStore 接口+内存回退+SQLite 持久化(NewSQLiteAuditStore 参数化 SQL+audit_logs 表)。
- server.go：Deps.AuditStore + Server.obs + 3 路由 GET /admin/observability/{metrics,trace,audit}；
  ObserveSpan/TraceSink 注入点；middleware logging recordMetric；audit 落库(仅 slog→Append)。
- 前端 Obs.tsx：metrics 接 4 张真实 StatCard+请求分布、trace 接 span 列表(展开 props/events)、
  audit 从后端拉(前端内存审计作回退)；api.ts/types.ts 加 obs 端点/类型(AdminObservabilityMetrics/
  AdminTraceSpan/AuditRecord.id 放宽)。成本指标不伪造(保留'—'+注明)。
- wire.go 收口：Deps.AuditStore 注入 gateway.NewSQLiteAuditStore(ctx, a.db)(审计真落库)，
  未注入回退内存。
- Docs/api-client.md §2/v0.6。

实机验证：三端点 200，metrics 返回 totalRequests:4/qps:4/perPath/statusCounts:{200:4}。
go build/vet/test 全绿 + 前端 tsc。trace 数据源对接(gw.TraceSink→loop tracer)因 gw/tracer
构造顺序留作可选后置(端点已就绪, 未对接 span 空)。

internal/services/gateway/audit.go

@@ -1,6 +1,7 @@
 package gateway
 
 import (
+	"context"
 	"net/http"
 	"time"
 
@@ -16,15 +17,23 @@ func (s *Server) auditClaims(r *http.Request, what, result string) {
 	if claims, ok := auth.ClaimsFromContext(r.Context()); ok {
 		who = claims.Sub + ":" + string(claims.Role)
 	}
-	s.audit(who, what, result)
+	s.audit(r.Context(), who, what, result)
 }
 
-// audit emits one structured audit record on the gateway logger.
-func (s *Server) audit(who, what, result string) {
+// audit emits one structured audit record on the gateway logger and persists it to
+// the configured AuditStore (falling back to an in-memory store when none is
+// injected). A persist failure is logged but never blocks the admin write, so a
+// broken audit store cannot take down the audit trail's primary job.
+func (s *Server) audit(ctx context.Context, who, what, result string) {
 	s.log.Info("admin audit",
 		"who", who,
 		"what", what,
 		"when", time.Now().UTC().Format(time.RFC3339),
 		"result", result,
 	)
+	if s.obs != nil && s.obs.audit != nil {
+		if err := s.obs.audit.Append(ctx, who, what, result, time.Now()); err != nil {
+			s.log.Error("admin audit persist", "err", err, "what", what)
+		}
+	}
 }