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/gateway_service.go

@@ -3789,7 +3789,7 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 	}
 
 	// Web Search 模拟：纯 web_search 请求时，直接调用搜索 API 构造响应
-	if account != nil && s.shouldEmulateWebSearch(ctx, account, parsed.Body) {
+	if account != nil && s.shouldEmulateWebSearch(ctx, account, parsed.GroupID, parsed.Body) {
 		return s.handleWebSearchEmulation(ctx, c, account, parsed)
 	}
 
@@ -7588,7 +7588,7 @@ func (s *GatewayService) recordUsageCore(ctx context.Context, input *recordUsage
 			upstreamModel = result.Model
 		}
 		usageLog.AccountStatsCost = resolveAccountStatsCost(
-			ctx, s.channelService,
+			ctx, s.channelService, s.billingService,
 			account.ID, *apiKey.GroupID, upstreamModel,
 			UsageTokens{
 				InputTokens:         result.Usage.InputTokens,