feat(api): expose the album endpoints

Eight routes under /albums covering the listing, the detail, the tracklist and
the owner-only writes, wired to the same validation filter and ErrorOr mapping
the playlists use.

backend/Musify.Api/Validators/Albums/AlbumReleaseYear.cs

@@ -0,0 +1,8 @@
+namespace Musify.Api.Validators.Albums;
+
+public static class AlbumReleaseYear
+{
+    public const int Earliest = 1877;
+
+    public static int Latest => DateTime.UtcNow.Year + 1;
+}

backend/Musify.Api/Validators/Albums/CreateAlbumRequestValidator.cs

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+using FluentValidation;
+using Musify.Api.DataTransferObjects.Albums;
+
+namespace Musify.Api.Validators.Albums;
+
+public sealed class CreateAlbumRequestValidator : AbstractValidator<CreateAlbumRequest>
+{
+    public CreateAlbumRequestValidator()
+    {
+        RuleFor(x => x.Title)
+            .NotEmpty()
+            .MaximumLength(200);
+
+        RuleFor(x => x.Description)
+            .MaximumLength(256);
+
+        RuleFor(x => x.ReleaseYear)
+            .InclusiveBetween(AlbumReleaseYear.Earliest, AlbumReleaseYear.Latest)
+            .When(x => x.ReleaseYear.HasValue);
+    }
+}

backend/Musify.Api/Validators/Albums/UpdateAlbumRequestValidator.cs

@@ -0,0 +1,21 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.Albums;
+
+namespace Musify.Api.Validators.Albums;
+
+public sealed class UpdateAlbumRequestValidator : AbstractValidator<UpdateAlbumRequest>
+{
+    public UpdateAlbumRequestValidator()
+    {
+        RuleFor(x => x.NewTitle)
+            .NotEmpty()
+            .MaximumLength(200);
+
+        RuleFor(x => x.NewDescription)
+            .MaximumLength(256);
+
+        RuleFor(x => x.NewReleaseYear)
+            .InclusiveBetween(AlbumReleaseYear.Earliest, AlbumReleaseYear.Latest)
+            .When(x => x.NewReleaseYear.HasValue);
+    }
+}