feat(server/m7-353): coverart.FindSidecar shared helper + tests

internal/coverart/sidecar.go

@@ -0,0 +1,35 @@
+// Package coverart finds and fetches album cover artwork.
+//
+// Two-layer model: a directory-walking sidecar lookup (cover.jpg / folder.jpg
+// next to the audio files) and a MusicBrainz Cover Art Archive (MBCAA)
+// fallback for albums with an MBID but no sidecar. The Enricher orchestrates
+// the two; the API and Subsonic packages reuse the sidecar helper directly.
+package coverart
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// SidecarNames is the lookup order for cover art living next to audio files.
+// Matches common library conventions: cover.* wins over folder.*; JPEG wins
+// over PNG when both are present.
+var SidecarNames = []string{
+	"cover.jpg", "cover.jpeg", "cover.png",
+	"folder.jpg", "folder.jpeg", "folder.png",
+}
+
+// FindSidecar returns the absolute path to a sidecar cover image in
+// albumDir, or "" when none of the conventional names are present as a
+// regular file. Directories named like sidecar files are skipped.
+func FindSidecar(albumDir string) string {
+	for _, name := range SidecarNames {
+		candidate := filepath.Join(albumDir, name)
+		info, err := os.Stat(candidate)
+		if err != nil || info.IsDir() {
+			continue
+		}
+		return candidate
+	}
+	return ""
+}