refactor(server): unify stream URL builders + MIME tables + cover-path helper

Closes Scribe #614, #615, server half of #616 surfaced by the 2026-06-04 divergent-provider audit.

- streamURL helper now used everywhere /api/tracks/{id}/stream is built (was inline concat in playlists.go and cast_token.go); add streamURLWithExt for the .ext cast variant.

- audioContentType in media.go is the canonical file_format -> MIME lookup; mimeForFormat in cast_token.go is now a thin wrapper that overrides the unknown-format fallback to audio/mpeg (Sonos rejects octet-stream). Adds mpeg/vorbis/wave aliases. Subsonic's contentTypeForFormat stays frozen per docs.

- coverart.ResolveAlbumPath extracted; api and subsonic both delegate to it.

internal/coverart/sidecar.go

@@ -7,8 +7,11 @@
 package coverart
 
 import (
+	"context"
 	"os"
 	"path/filepath"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
 // SidecarNames is the lookup order for cover art living next to audio files.
@@ -33,3 +36,24 @@ func FindSidecar(albumDir string) string {
 	}
 	return ""
 }
+
+// ResolveAlbumPath returns the on-disk path to an album's cover image,
+// preferring the explicit album.cover_art_path when set and the file
+// exists, falling back to a sidecar (cover.jpg / folder.jpg) next to the
+// first track in the album's directory. "" means no art was found.
+//
+// Shared by internal/api/media.go (browser endpoint) and
+// internal/subsonic/stream.go (Subsonic endpoint); they were byte-identical
+// duplicates before extraction.
+func ResolveAlbumPath(ctx context.Context, q *dbq.Queries, album dbq.Album) string {
+	if album.CoverArtPath != nil && *album.CoverArtPath != "" {
+		if _, err := os.Stat(*album.CoverArtPath); err == nil {
+			return *album.CoverArtPath
+		}
+	}
+	tracks, err := q.ListTracksByAlbum(ctx, dbq.ListTracksByAlbumParams{AlbumID: album.ID})
+	if err != nil || len(tracks) == 0 {
+		return ""
+	}
+	return FindSidecar(filepath.Dir(tracks[0].FilePath))
+}