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/setting_service.go

@@ -18,6 +18,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/imroc/req/v3"
+	"github.com/redis/go-redis/v9"
 	"golang.org/x/sync/singleflight"
 )
 
@@ -106,6 +107,7 @@ type SettingService struct {
 	cfg                   *config.Config
 	onUpdate              func() // Callback when settings are updated (for cache invalidation)
 	version               string // Application version
+	webSearchRedis        *redis.Client // optional: Redis client for web search quota tracking
 }
 
 // NewSettingService 创建系统设置服务实例
@@ -1217,6 +1219,14 @@ func (s *SettingService) parseSettings(settings map[string]string) *SystemSettin
 	result.EnableMetadataPassthrough = settings[SettingKeyEnableMetadataPassthrough] == "true"
 	result.EnableCCHSigning = settings[SettingKeyEnableCCHSigning] == "true"
 
+	// Web search emulation: quick enabled check from the JSON config
+	if raw := settings[SettingKeyWebSearchEmulationConfig]; raw != "" {
+		var wsCfg WebSearchEmulationConfig
+		if err := json.Unmarshal([]byte(raw), &wsCfg); err == nil {
+			result.WebSearchEmulationEnabled = wsCfg.Enabled && len(wsCfg.Providers) > 0
+		}
+	}
+
 	return result
 }