fix: resolve remaining lint errors for upstream CI

- Fix errcheck: brave.go resp.Body.Close, manager_test.go Encode
- Fix gofmt: payment_config_service.go
- Fix unused: use shouldFallbackGeminiModel (with modelName param) in handler

backend/internal/pkg/websearch/brave.go

@@ -62,7 +62,7 @@ func (b *BraveProvider) Search(ctx context.Context, req SearchRequest) (*SearchR
 	if err != nil {
 		return nil, fmt.Errorf("brave: request failed: %w", err)
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseSize))
 	if err != nil {