Centralize remaining runtime constants and add precedence/default fallback coverage

internal/artifact/reader.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"errors"
 	"fmt"
+	"gitea.maximumdirect.net/eric/scriptorium/internal/defaults"
 	"gitea.maximumdirect.net/eric/scriptorium/internal/domain"
 	"mime"
 	"os"
@@ -67,7 +68,7 @@ func (r *inlineReader) Read(ctx context.Context, ref domain.ArtifactRef) (*domai
 
 	body := []byte(ref.Body)
 	return &domain.Artifact{
-		ContentType: "text/plain",
+		ContentType: defaults.ContentTypeTextPlain,
 		Body:        body,
 		Size:        int64(len(body)),
 		Hash:        fmt.Sprintf("%x", sha256.Sum256(body)),
@@ -95,7 +96,7 @@ func (r *fileReader) Read(ctx context.Context, ref domain.ArtifactRef) (*domain.
 
 	contentType := mime.TypeByExtension(filepath.Ext(ref.URI))
 	if contentType == "" {
-		contentType = "text/plain" // Default
+		contentType = defaults.ContentTypeTextPlain
 	}
 
 	return &domain.Artifact{