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/api/admin/channels.ts

@@ -41,6 +41,7 @@ export interface Channel {
   status: string
   billing_model_source: string // "requested" | "upstream"
   restrict_models: boolean
+  features_config?: Record<string, unknown>
   group_ids: number[]
   model_pricing: ChannelModelPricing[]
   model_mapping: Record<string, Record<string, string>> // platform → {src→dst}
@@ -56,6 +57,7 @@ export interface CreateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
+  features_config?: Record<string, unknown>
 }
 
 export interface UpdateChannelRequest {
@@ -67,6 +69,7 @@ export interface UpdateChannelRequest {
   model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
+  features_config?: Record<string, unknown>
 }
 
 interface PaginatedResponse<T> {

frontend/src/api/admin/settings.ts

@@ -482,6 +482,42 @@ export async function updateBetaPolicySettings(
   return data
 }
 
+// --- Web Search Emulation Config ---
+
+export interface WebSearchProviderConfig {
+  type: 'brave' | 'tavily'
+  api_key: string
+  api_key_configured: boolean
+  priority: number
+  quota_limit: number
+  quota_refresh_interval: 'daily' | 'weekly' | 'monthly'
+  quota_used?: number
+  proxy_id: number | null
+  expires_at: number | null
+}
+
+export interface WebSearchEmulationConfig {
+  enabled: boolean
+  providers: WebSearchProviderConfig[]
+}
+
+export async function getWebSearchEmulationConfig(): Promise<WebSearchEmulationConfig> {
+  const { data } = await apiClient.get<WebSearchEmulationConfig>(
+    '/admin/settings/web-search-emulation'
+  )
+  return data
+}
+
+export async function updateWebSearchEmulationConfig(
+  config: WebSearchEmulationConfig
+): Promise<WebSearchEmulationConfig> {
+  const { data } = await apiClient.put<WebSearchEmulationConfig>(
+    '/admin/settings/web-search-emulation',
+    config
+  )
+  return data
+}
+
 export const settingsAPI = {
   getSettings,
   updateSettings,
@@ -497,7 +533,9 @@ export const settingsAPI = {
   getRectifierSettings,
   updateRectifierSettings,
   getBetaPolicySettings,
-  updateBetaPolicySettings
+  updateBetaPolicySettings,
+  getWebSearchEmulationConfig,
+  updateWebSearchEmulationConfig
 }
 
 export default settingsAPI