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/channel.go

@@ -49,6 +49,21 @@ type Channel struct {
 	ModelPricing []ChannelModelPricing
 	// 渠道级模型映射（按平台分组：platform → {src→dst}）
 	ModelMapping map[string]map[string]string
+	// 渠道特性配置（如 {"web_search_emulation": {"anthropic": true}}）
+	FeaturesConfig map[string]any
+}
+
+// IsWebSearchEmulationEnabled 返回该渠道是否为指定平台启用了 web search 模拟。
+func (c *Channel) IsWebSearchEmulationEnabled(platform string) bool {
+	if c == nil || c.FeaturesConfig == nil {
+		return false
+	}
+	wse, ok := c.FeaturesConfig[featureKeyWebSearchEmulation].(map[string]any)
+	if !ok {
+		return false
+	}
+	enabled, ok := wse[platform].(bool)
+	return ok && enabled
 }
 
 // ChannelModelPricing 渠道模型定价条目