feat: add PlaylistRepresentable protocol, conform Playlist

Music/Models/Playlist.swift

@@ -15,6 +15,10 @@ nonisolated extension Playlist: FetchableRecord, MutablePersistableRecord {
     }
 }
 
+extension Playlist: PlaylistRepresentable {
+    var isSmartPlaylist: Bool { false }
+}
+
 #if DEBUG
 extension Playlist {
     static func fixture(

Music/Protocols/PlaylistRepresentable.swift

@@ -0,0 +1,7 @@
+import Foundation
+
+protocol PlaylistRepresentable: Identifiable, Hashable, Sendable {
+    var id: Int64? { get }
+    var name: String { get }
+    var isSmartPlaylist: Bool { get }
+}