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)

backend/internal/service/account.go

@@ -969,7 +969,7 @@ func (a *Account) IsOveragesEnabled() bool {
 	return false
 }
 
-// IsOpenAIPassthroughEnabled 返回 OpenAI 账号是否启用“自动透传（仅替换认证）”。
+// IsOpenAIPassthroughEnabled 返回 OpenAI 账号是否启用"自动透传（仅替换认证）"。
 //
 // 新字段：accounts.extra.openai_passthrough。
 // 兼容字段：accounts.extra.openai_oauth_passthrough（历史 OAuth 开关）。
@@ -1133,7 +1133,7 @@ func (a *Account) ResolveOpenAIResponsesWebSocketV2Mode(defaultMode string) stri
 	return resolvedDefault
 }
 
-// IsOpenAIWSForceHTTPEnabled 返回账号级“强制 HTTP”开关。
+// IsOpenAIWSForceHTTPEnabled 返回账号级"强制 HTTP"开关。
 // 字段：accounts.extra.openai_ws_force_http。
 func (a *Account) IsOpenAIWSForceHTTPEnabled() bool {
 	if a == nil || !a.IsOpenAI() || a.Extra == nil {
@@ -1158,7 +1158,7 @@ func (a *Account) IsOpenAIOAuthPassthroughEnabled() bool {
 	return a != nil && a.IsOpenAIOAuth() && a.IsOpenAIPassthroughEnabled()
 }
 
-// IsAnthropicAPIKeyPassthroughEnabled 返回 Anthropic API Key 账号是否启用“自动透传（仅替换认证）”。
+// IsAnthropicAPIKeyPassthroughEnabled 返回 Anthropic API Key 账号是否启用"自动透传（仅替换认证）"。
 // 字段：accounts.extra.anthropic_passthrough。
 // 字段缺失或类型不正确时，按 false（关闭）处理。
 func (a *Account) IsAnthropicAPIKeyPassthroughEnabled() bool {
@@ -1169,7 +1169,18 @@ func (a *Account) IsAnthropicAPIKeyPassthroughEnabled() bool {
 	return ok && enabled
 }
 
-// IsCodexCLIOnlyEnabled 返回 OpenAI OAuth 账号是否启用“仅允许 Codex 官方客户端”。
+// IsWebSearchEmulationEnabled 返回 Anthropic API Key 账号是否启用 web search 模拟。
+// 字段：accounts.extra.web_search_emulation。
+// 字段缺失或类型不正确时，按 false（关闭）处理。
+func (a *Account) IsWebSearchEmulationEnabled() bool {
+	if a == nil || a.Platform != PlatformAnthropic || a.Type != AccountTypeAPIKey || a.Extra == nil {
+		return false
+	}
+	enabled, ok := a.Extra[featureKeyWebSearchEmulation].(bool)
+	return ok && enabled
+}
+
+// IsCodexCLIOnlyEnabled 返回 OpenAI OAuth 账号是否启用"仅允许 Codex 官方客户端"。
 // 字段：accounts.extra.codex_cli_only。
 // 字段缺失或类型不正确时，按 false（关闭）处理。
 func (a *Account) IsCodexCLIOnlyEnabled() bool {