feat: image vault + cover-art API (upload/serve endpoints, ImagePath metadata link)

DeepDrftAPI/Models/UpdateTrackMetadataRequest.cs

@@ -4,9 +4,15 @@ namespace DeepDrftAPI.Models;
 /// Body of <c>PUT api/track/meta/{id}</c>. Metadata-only — EntryKey is immutable and never
 /// travels over this surface.
 /// </summary>
+/// <remarks>
+/// <paramref name="ImagePath"/> follows tri-state semantics distinct from the other optional
+/// fields: <c>null</c> leaves the existing value unchanged, an empty string clears it, and a
+/// non-empty value is the images-vault entry key to link.
+/// </remarks>
 public record UpdateTrackMetadataRequest(
     string TrackName,
     string Artist,
     string? Album,
     string? Genre,
-    DateOnly? ReleaseDate);
+    DateOnly? ReleaseDate,
+    string? ImagePath = null);