feat(moderation): route content moderation through configurable proxy server

Implements #2646: the risk-control content audit can now send OpenAI
Moderations requests through a proxy from IP Management - Proxy Servers.

Backend:
- ContentModerationConfig gains proxy_id (nil = direct, unchanged default)
- update semantics: null keeps, 0 clears, >0 selects (validated to exist)
- moderation calls build the client via the shared httpclient pool; proxy
  resolution failure surfaces as a moderation error and never silently
  falls back to direct connection
- proxy_id -> URL resolution cached 60s (single-entry, invalidated on
  config save) so the pre-block hot path does not hit the DB per request
- test-key endpoint accepts proxy_id too (null = saved config's proxy,
  0 = force direct), so input-key/saved-key tests exercise the same path
- proxy usage/inactivity logged (content_moderation.proxy_enabled /
  proxy_not_active) without leaking credentials

Frontend:
- ProxySelector in the risk-control basic settings tab, proxy list loaded
  non-blockingly; save and test payloads carry proxy_id; zh/en i18n

backend/internal/handler/admin/content_moderation_handler.go

@@ -20,10 +20,12 @@ func NewContentModerationHandler(svc *service.ContentModerationService) *Content
 }
 
 type contentModerationConfigRequest struct {
-	Enabled              *bool               `json:"enabled"`
-	Mode                 *string             `json:"mode"`
-	BaseURL              *string             `json:"base_url"`
-	Model                *string             `json:"model"`
+	Enabled *bool   `json:"enabled"`
+	Mode    *string `json:"mode"`
+	BaseURL *string `json:"base_url"`
+	Model   *string `json:"model"`
+	// 审计请求使用的代理服务器：null 不修改；0 清除（直连）；>0 指定代理。
+	ProxyID              *int64              `json:"proxy_id"`
 	APIKey               *string             `json:"api_key"`
 	APIKeys              *[]string           `json:"api_keys"`
 	APIKeysMode          string              `json:"api_keys_mode"`
@@ -60,6 +62,7 @@ type contentModerationAPIKeyTestRequest struct {
 	BaseURL   string   `json:"base_url"`
 	Model     string   `json:"model"`
 	TimeoutMS int      `json:"timeout_ms"`
+	ProxyID   *int64   `json:"proxy_id"`
 	Prompt    string   `json:"prompt"`
 	Images    []string `json:"images"`
 }
@@ -88,6 +91,7 @@ func (h *ContentModerationHandler) UpdateConfig(c *gin.Context) {
 		Mode:                           req.Mode,
 		BaseURL:                        req.BaseURL,
 		Model:                          req.Model,
+		ProxyID:                        req.ProxyID,
 		APIKey:                         req.APIKey,
 		APIKeys:                        req.APIKeys,
 		APIKeysMode:                    req.APIKeysMode,
@@ -134,6 +138,7 @@ func (h *ContentModerationHandler) TestAPIKeys(c *gin.Context) {
 		BaseURL:   req.BaseURL,
 		Model:     req.Model,
 		TimeoutMS: req.TimeoutMS,
+		ProxyID:   req.ProxyID,
 		Prompt:    req.Prompt,
 		Images:    req.Images,
 	})