feat(gateway): 敏感操作审计日志（T4.B）

- store.AuditLog 表(sundynix_audit_log) + AppendAudit/ListAudit
- middleware.Audit(db)：只审计变更类(POST/PUT/DELETE/PATCH)，收尾 best-effort
  落库(独立超时 ctx，失败静默不拖垮主流程)；挂管理组 + prompt 激活/撤销 + HITL 审批
- GET /api/v1/admin/audit：倒序审计流(limit/offset 翻页)
- live：PUT pricing / POST prompts/deactivate 留痕(actor/path/status/ip)，GET 不记
- DEPTH_ROADMAP T4.B：overview✅ + audit✅ 打勾

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-gateway/internal/middleware/audit.go

@@ -0,0 +1,40 @@
+package middleware
+
+import (
+	"context"
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/sundynix/sundynix-gateway/internal/store"
+)
+
+// Audit 记录敏感操作留痕：仅对变更类请求（POST/PUT/DELETE）在收尾时 best-effort 落库。
+// 挂在受保护/管理组上即可自动覆盖模型/密钥/prompt/审批等变更，无需在每个 handler 里手写。
+// 写库用独立超时 context（不受请求取消影响），失败静默——审计不能拖垮主流程。
+func Audit(db *store.Postgres) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Next() // 先跑 handler，拿到最终状态码
+
+		switch c.Request.Method {
+		case "POST", "PUT", "DELETE", "PATCH":
+		default:
+			return // 只审计变更类操作
+		}
+
+		actor, _ := c.Get(CtxUserID)
+		uid, _ := actor.(string)
+
+		entry := &store.AuditLog{
+			Actor:  uid,
+			Action: c.Request.Method,
+			Route:  c.FullPath(),
+			Path:   c.Request.URL.Path,
+			Status: c.Writer.Status(),
+			IP:     c.ClientIP(),
+		}
+		ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+		defer cancel()
+		_ = db.AppendAudit(ctx, entry)
+	}
+}