feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

backend/internal/handler/openai_images.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"net/http"
+	"strconv"
 	"strings"
 	"time"
 
@@ -108,7 +109,10 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 
 	if err := h.billingCacheService.CheckBillingEligibility(c.Request.Context(), apiKey.User, apiKey, apiKey.Group, subscription); err != nil {
 		reqLog.Info("openai.images.billing_eligibility_check_failed", zap.Error(err))
-		status, code, message := billingErrorDetails(err)
+		status, code, message, retryAfter := billingErrorDetails(err)
+		if retryAfter > 0 {
+			c.Header("Retry-After", strconv.Itoa(retryAfter))
+		}
 		h.handleStreamingAwareError(c, status, code, message, streamStarted)
 		return
 	}