feat: WebSearch tri-state, account stats pricing fix, quota cache fix, usage tooltip

WebSearch tri-state switch:
- Account-level web_search_emulation changed from bool to tri-state
  string: "default" (follow channel) / "enabled" / "disabled"
- shouldEmulateWebSearch checks channel config when account is "default"
- SQL migration converts old bool values
- Frontend select replaces toggle in Edit/CreateAccountModal

Account stats pricing:
- resolveAccountStatsCost uses upstream model (post-mapping) for matching
- Priority: custom rules → model pricing file (when toggle on) → default
- Custom rules always configurable, independent of toggle
- Account ID field changed to searchable selector filtered by platform
- Description updated to reflect new behavior

Quota notification cache fix:
- CheckAccountQuotaAfterIncrement fetches real-time account from DB
- Reconstructs pre-increment usage for accurate threshold crossing detection
- New AccountQuotaReader interface (minimal: GetByID only)

Usage tooltip:
- Per-request/image billing shows per-request price instead of $0 token price
- Token billing continues to show input/output price per million tokens

backend/internal/service/account.go

@@ -1169,15 +1169,30 @@ func (a *Account) IsAnthropicAPIKeyPassthroughEnabled() bool {
 	return ok && enabled
 }
 
-// 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 {
+// WebSearch 模拟三态常量
+const (
+	WebSearchModeDefault  = "default"  // 跟随渠道配置
+	WebSearchModeEnabled  = "enabled"  // 强制开启
+	WebSearchModeDisabled = "disabled" // 强制关闭
+)
+
+// GetWebSearchEmulationMode 返回账号的 WebSearch 模拟模式。
+// 三态：default（跟随渠道）/ enabled（强制开启）/ disabled（强制关闭）。
+// 旧 bool 值需通过 SQL 迁移脚本转换，Go 代码不做兼容。
+func (a *Account) GetWebSearchEmulationMode() string {
 	if a == nil || a.Platform != PlatformAnthropic || a.Type != AccountTypeAPIKey || a.Extra == nil {
-		return false
+		return WebSearchModeDefault
+	}
+	mode, ok := a.Extra[featureKeyWebSearchEmulation].(string)
+	if !ok {
+		return WebSearchModeDefault
+	}
+	switch mode {
+	case WebSearchModeEnabled, WebSearchModeDisabled:
+		return mode
+	default:
+		return WebSearchModeDefault
 	}
-	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"。