fix(openai): Mobile RT 账号隐私设置失败

1. CreateAccount 补齐 OpenAI OAuth 隐私入口（与 BatchCreate 对齐）
2. disableOpenAITraining 请求头修正：覆盖 ImpersonateChrome() 的
   浏览器导航默认头（accept: text/html, sec-fetch-mode: navigate），
   改为 API 请求语义（Accept: application/json, sec-fetch-mode: cors），
   避免 Cloudflare 将 PATCH API 请求误判为异常导航流量而拦截

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/admin_service.go

@@ -1642,16 +1642,29 @@ func (s *adminServiceImpl) CreateAccount(ctx context.Context, input *CreateAccou
 		}
 	}
 
-	// Antigravity OAuth 账号：创建后异步设置隐私
-	if account.Platform == PlatformAntigravity && account.Type == AccountTypeOAuth {
-		go func() {
-			defer func() {
-				if r := recover(); r != nil {
-					slog.Error("create_account_antigravity_privacy_panic", "account_id", account.ID, "recover", r)
-				}
+	// OAuth 账号：创建后异步设置隐私。
+	// 使用 Ensure（幂等）而非 Force：新建账号 Extra 为空时效果相同，但更安全。
+	if account.Type == AccountTypeOAuth {
+		switch account.Platform {
+		case PlatformOpenAI:
+			go func() {
+				defer func() {
+					if r := recover(); r != nil {
+						slog.Error("create_account_openai_privacy_panic", "account_id", account.ID, "recover", r)
+					}
+				}()
+				s.EnsureOpenAIPrivacy(context.Background(), account)
 			}()
-			s.EnsureAntigravityPrivacy(context.Background(), account)
-		}()
+		case PlatformAntigravity:
+			go func() {
+				defer func() {
+					if r := recover(); r != nil {
+						slog.Error("create_account_antigravity_privacy_panic", "account_id", account.ID, "recover", r)
+					}
+				}()
+				s.EnsureAntigravityPrivacy(context.Background(), account)
+			}()
+		}
 	}
 
 	return account, nil