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/components/account/CreateAccountModal.vue

@@ -2325,6 +2325,22 @@
         </div>
       </div>
 
+      <!-- Anthropic API Key: Web Search Emulation -->
+      <div
+        v-if="form.platform === 'anthropic' && accountCategory === 'apikey'"
+        class="border-t border-gray-200 pt-4 dark:border-dark-600"
+      >
+        <div class="flex items-center justify-between">
+          <div>
+            <label class="input-label mb-0">{{ t('admin.accounts.anthropic.webSearchEmulation') }}</label>
+            <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+              {{ t('admin.accounts.anthropic.webSearchEmulationDesc') }}
+            </p>
+          </div>
+          <Toggle v-model="webSearchEmulationEnabled" />
+        </div>
+      </div>
+
       <!-- OpenAI OAuth Codex 官方客户端限制开关 -->
       <div
         v-if="form.platform === 'openai' && accountCategory === 'oauth-based'"
@@ -2830,6 +2846,7 @@ import ConfirmDialog from '@/components/common/ConfirmDialog.vue'
 import Select from '@/components/common/Select.vue'
 import Icon from '@/components/icons/Icon.vue'
 import ProxySelector from '@/components/common/ProxySelector.vue'
+import Toggle from '@/components/common/Toggle.vue'
 import GroupSelector from '@/components/common/GroupSelector.vue'
 import ModelWhitelistSelector from '@/components/account/ModelWhitelistSelector.vue'
 import QuotaLimitCard from '@/components/account/QuotaLimitCard.vue'
@@ -2980,6 +2997,7 @@ const openaiOAuthResponsesWebSocketV2Mode = ref<OpenAIWSMode>(OPENAI_WS_MODE_OFF
 const openaiAPIKeyResponsesWebSocketV2Mode = ref<OpenAIWSMode>(OPENAI_WS_MODE_OFF)
 const codexCLIOnlyEnabled = ref(false)
 const anthropicPassthroughEnabled = ref(false)
+const webSearchEmulationEnabled = ref(false)
 const mixedScheduling = ref(false) // For antigravity accounts: enable mixed scheduling
 const allowOverages = ref(false) // For antigravity accounts: enable AI Credits overages
 const antigravityAccountType = ref<'oauth' | 'upstream'>('oauth') // For antigravity: oauth or upstream
@@ -3307,6 +3325,7 @@ watch(
     }
     if (newPlatform !== 'anthropic') {
       anthropicPassthroughEnabled.value = false
+      webSearchEmulationEnabled.value = false
     }
     // Reset OAuth states
     oauth.resetState()
@@ -3326,6 +3345,7 @@ watch(
     }
     if (platform !== 'anthropic' || category !== 'apikey') {
       anthropicPassthroughEnabled.value = false
+      webSearchEmulationEnabled.value = false
     }
   }
 )
@@ -3690,6 +3710,7 @@ const resetForm = () => {
   openaiAPIKeyResponsesWebSocketV2Mode.value = OPENAI_WS_MODE_OFF
   codexCLIOnlyEnabled.value = false
   anthropicPassthroughEnabled.value = false
+  webSearchEmulationEnabled.value = false
   // Reset quota control state
   windowCostEnabled.value = false
   windowCostLimit.value = null
@@ -3777,6 +3798,11 @@ const buildAnthropicExtra = (base?: Record<string, unknown>): Record<string, unk
   } else {
     delete extra.anthropic_passthrough
   }
+  if (webSearchEmulationEnabled.value) {
+    extra.web_search_emulation = true
+  } else {
+    delete extra.web_search_emulation
+  }
 
   return Object.keys(extra).length > 0 ? extra : undefined
 }