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/handler/gateway_handler.go

@@ -248,6 +248,9 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 		return
 	}
 
+	// 设置请求所属分组 ID（用于渠道级功能判断，如 WebSearch 模拟）
+	parsedReq.GroupID = apiKey.GroupID
+
 	// 计算粘性会话hash
 	parsedReq.SessionContext = &service.SessionContext{
 		ClientIP:  ip.GetClientIP(c),