feat: apikey 账号支持请求头覆写（Anthropic/OpenAI）

- 账号 credentials 新增 header_override_enabled / header_overrides，
  仅对 anthropic/openai 平台的 api_key 账号生效
- 转发前对同名请求头做大小写不敏感覆盖（EqualFold 全量删除后按
  wire casing 写入单值），值为空的条目视为占位不参与覆盖
- 覆盖全部出站路径：/v1/messages（标准+透传）、count_tokens、
  /v1/responses、chat completions、embeddings、images、全部 WS 模式、
  账号测试/探测、上游模型列表
- 创建/编辑/批量更新统一校验：RFC 7230 名称格式、去重、长度/条目上限、
  24 个禁止覆写头（认证/连接控制/accept-encoding/sec-websocket-*/
  会话隔离头），应用时二次防御过滤
- 前端三个账号弹窗新增开关+键值行编辑器+模板按钮（Claude Code CLI /
  Codex CLI 标准头，值为空），本地校验与后端规则对齐，i18n 中英文

frontend/src/i18n/locales/en.ts

@@ -3674,6 +3674,22 @@ export default {
       interceptWarmupRequests: 'Intercept Warmup Requests',
       interceptWarmupRequestsDesc:
         'When enabled, warmup requests like title generation will return mock responses without consuming upstream tokens',
+      headerOverride: {
+        title: 'Header Override',
+        hint: 'Override same-named request headers on forwarding (case-insensitive)',
+        info: 'Applies to outbound requests of this account only: configured headers override client/gateway-generated headers of the same name before forwarding. Auth headers (authorization, x-api-key) and connection-control headers cannot be overridden.',
+        namePlaceholder: 'Header name (e.g. user-agent)',
+        valuePlaceholder: 'Override value (leave empty to skip)',
+        addRow: 'Add Header',
+        fillTemplate: 'Fill Template',
+        emptyValueHint: 'Rows with an empty value are placeholders and do not override anything.',
+        bulkDisableHint: 'Saving will disable header override and clear existing configuration on the selected accounts.',
+        invalidName: 'Invalid header name (only letters, digits and !#$%&\'*+-.^_`|~ are allowed)',
+        blockedName: 'This header cannot be overridden (auth and connection-control headers are managed by the system)',
+        duplicateName: 'Duplicate header name (matching is case-insensitive)',
+        invalidValue: 'Invalid header value (control characters are not allowed; max length 8192)',
+        tooManyEntries: 'Too many header override entries (max 64)'
+      },
       autoPauseOnExpired: 'Auto Pause On Expired',
       autoPauseOnExpiredDesc: 'When enabled, the account will auto pause scheduling after it expires',
 	  autoPause5hThreshold: '5h Usage Threshold (%)',

frontend/src/i18n/locales/zh.ts

@@ -3842,6 +3842,22 @@ export default {
       errorCodeExists: '该错误码已被选中',
       interceptWarmupRequests: '拦截预热请求',
       interceptWarmupRequestsDesc: '启用后，标题生成等预热请求将返回 mock 响应，不消耗上游 token',
+      headerOverride: {
+        title: '请求头覆写',
+        hint: '转发时用配置值覆盖同名请求头（不区分大小写）',
+        info: '仅对本账号的出站请求生效：配置的请求头会在转发前覆盖客户端/网关生成的同名头。认证头（authorization、x-api-key）与连接控制头不允许覆写。',
+        namePlaceholder: '请求头名称（如 user-agent）',
+        valuePlaceholder: '覆写值（留空表示不覆写）',
+        addRow: '添加请求头',
+        fillTemplate: '填入模板',
+        emptyValueHint: '值留空的行不会参与覆盖，仅作为待填写的占位。',
+        bulkDisableHint: '保存后将关闭所选账号的请求头覆写并清空已有配置。',
+        invalidName: '请求头名称格式不正确（仅允许字母、数字和 !#$%&\'*+-.^_`|~ 字符）',
+        blockedName: '该请求头不允许覆写（认证头与连接控制头由系统管理）',
+        duplicateName: '存在重复的请求头名称（匹配不区分大小写）',
+        invalidValue: '请求头值不合法（不允许控制字符，长度不超过 8192）',
+        tooManyEntries: '请求头覆写条目过多（最多 64 条）'
+      },
       autoPauseOnExpired: '过期自动暂停调度',
       autoPauseOnExpiredDesc: '启用后，账号过期将自动暂停调度',
 	  autoPause5hThreshold: '5h 用量阈值(%)',