feat(gateway): add web search emulation for Anthropic API Key accounts

Inject web search capability for Claude Console (API Key) accounts that
don't natively support Anthropic's web_search tool. When a pure
web_search request is detected, the gateway calls Brave Search or Tavily
API directly and constructs an Anthropic-protocol-compliant SSE/JSON
response without forwarding to upstream.

Backend:
- New `pkg/websearch/` SDK: Brave and Tavily provider implementations
  with io.LimitReader, proxy support, and Redis-based quota tracking
  (Lua atomic INCR + TTL, DECR rollback on failure)
- Global config via `settings.web_search_emulation_config` (JSON) with
  in-process cache + singleflight, input validation, API key merge on
  save, and sanitized API responses
- Channel-level toggle via `channels.features_config` JSONB column
  (DB migration 101)
- Account-level toggle via `accounts.extra.web_search_emulation`
- Request interception in `Forward()` with SSE streaming response
  construction using json.Marshal (no manual string concatenation)
- Manager hot-reload: `RebuildWebSearchManager()` called on config save
  and startup via `SetWebSearchRedisClient()`
- 70 unit tests covering providers, manager, config validation,
  sanitization, tool detection, query extraction, and response building

Frontend:
- Settings → Gateway tab: Web Search Emulation config card with global
  toggle, provider list (add/remove, API key, priority, quota, proxy)
- Channels → Anthropic tab: web search emulation toggle with global
  state linkage (disabled when global off)
- Account Create/Edit modals: web search emulation toggle for API Key
  type with Toggle component
- Full i18n coverage (zh + en)

frontend/src/i18n/locales/zh.ts

@@ -1915,6 +1915,9 @@ export default {
         defaultPerRequestPrice: '默认单次价格（未命中层级时使用）',
         defaultImagePrice: '默认图片价格（未命中层级时使用）',
         platformConfig: '平台配置',
+        webSearchEmulation: 'Web Search 模拟',
+        webSearchEmulationHint: '⚠️ 开启后该渠道下所有 Anthropic 分组的账号将自动拦截 web_search 请求，请谨慎操作',
+        webSearchEmulationGlobalDisabled: '请先在系统设置 → 网关 → Web Search 模拟中启用全局开关',
         basicSettings: '基础设置',
         addPlatform: '添加平台',
         noPlatforms: '点击"添加平台"开始配置渠道',
@@ -2472,7 +2475,10 @@ export default {
       anthropic: {
         apiKeyPassthrough: '自动透传（仅替换认证）',
         apiKeyPassthroughDesc:
-          '仅对 Anthropic API Key 生效。开启后，messages/count_tokens 请求将透传上游并仅替换认证，保留计费/并发/审计及必要安全过滤；关闭即可回滚到现有兼容链路。'
+          '仅对 Anthropic API Key 生效。开启后，messages/count_tokens 请求将透传上游并仅替换认证，保留计费/并发/审计及必要安全过滤；关闭即可回滚到现有兼容链路。',
+        webSearchEmulation: 'Web Search 模拟',
+        webSearchEmulationDesc:
+          '为该 API Key 账号启用 web search 模拟。客户端发送纯 web_search 请求时，由网关调用第三方搜索 API 并构造响应返回。',
       },
       modelRestriction: '模型限制（可选）',
       modelWhitelist: '模型白名单',
@@ -4520,6 +4526,31 @@ export default {
         cchSigning: 'CCH 签名',
         cchSigningHint: '对转发请求的 billing header 进行 CCH 哈希签名。关闭时保留原始占位符。',
       },
+      webSearchEmulation: {
+        title: 'Web Search 模拟',
+        description: '为不原生支持搜索的 Anthropic API Key 账号注入 web search 能力',
+        enabled: '启用 Web Search 模拟',
+        enabledHint: '全局开关。关闭后所有渠道和账号的 web search 模拟均不生效。',
+        providers: '搜索服务商',
+        addProvider: '添加服务商',
+        providerType: '服务商类型',
+        apiKey: 'API Key',
+        apiKeyPlaceholder: '输入 API Key',
+        apiKeyConfigured: '已配置',
+        priority: '优先级',
+        priorityHint: '数值越小优先级越高',
+        quotaLimit: '配额上限',
+        quotaLimitHint: '0 表示无限制',
+        quotaRefreshInterval: '刷新周期',
+        quotaUsed: '已使用',
+        proxy: '代理',
+        expiresAt: '过期时间',
+        removeProvider: '删除',
+        daily: '每日',
+        weekly: '每周',
+        monthly: '每月',
+        noProviders: '未配置搜索服务商',
+      },
       site: {
         title: '站点设置',
         description: '自定义站点品牌',