Centralize link URL validation

internal/publish/links.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"gitea.maximumdirect.net/eric/distributor/internal/config"
+	"gitea.maximumdirect.net/eric/distributor/internal/link"
 	"gitea.maximumdirect.net/eric/distributor/internal/state"
 )
 
@@ -13,6 +14,9 @@ func PlanLinks(req Request, outputs []Output) ([]Output, string, error) {
 	if req.Links == nil {
 		return outputs, "", nil
 	}
+	if err := link.ValidateHTTPURL(req.Links.BaseURL); err != nil {
+		return nil, "", fmt.Errorf("link base URL: %w", err)
+	}
 	linked := make([]Output, 0, len(outputs))
 	for _, output := range outputs {
 		outputURL, err := OutputURL(req.Links.BaseURL, req.DestinationBundlePath, output.DestinationPath)