Make Discover and upload genres dynamic

Discover used a hardcoded list of genre tiles that searched track titles,
and the upload form kept its own copy of the genre list and compatibility
rules. Both now come from the backend.

Backend:
- GET /genres returns genres with at least one track and their counts.
- GET /genres/available returns every assignable genre with the genres
  it cannot be combined with, so the rules live only in
  GenreCompatibility.
- GET /tracks accepts ?genre= (case-insensitive, 400 on unknown values).

web-player:
- Discover tiles are built from /genres; each opens /explore?genre=X, a
  song list filtered by that tag. Unknown genres fall back to a label
  derived from their name.
- The upload form loads /genres/available, disables incompatible genres
  as you pick, and the server validates before reserving upload URLs.
- lib/data/trackGenres.ts is removed; its helpers merge into
  lib/data/genres.ts.
- Regenerate the typed API client.

backend/Tests/Musify.Application.Tests/Genres/GetAvailableGenresQueryHandlerTests.cs

@@ -0,0 +1,32 @@
+using Musify.Application.Genres;
+using Musify.Domain.ValueObjects;
+using Xunit;
+
+namespace Musify.Application.Tests.Genres
+{
+    public sealed class GetAvailableGenresQueryHandlerTests
+    {
+        private static async Task<IReadOnlyList<AvailableGenreResponse>> RunAsync() =>
+            await new GetAvailableGenresQueryHandler().Handle(new GetAvailableGenresQuery(), TestContext.Current.CancellationToken);
+
+        [Fact]
+        public async Task Handle_ReturnsEveryGenre()
+        {
+            var result = await RunAsync();
+
+            Assert.Equal(Enum.GetValues<Genre>(), result.Select(r => r.Genre));
+        }
+
+        [Fact]
+        public async Task Handle_ListsIncompatibleGenresSymmetrically()
+        {
+            var result = await RunAsync();
+
+            var metal = result.Single(r => r.Genre == Genre.Metal);
+            var lofi = result.Single(r => r.Genre == Genre.Lofi);
+            Assert.Contains(Genre.Lofi, metal.IncompatibleWith);
+            Assert.Contains(Genre.Metal, lofi.IncompatibleWith);
+            Assert.Empty(result.Single(r => r.Genre == Genre.Pop).IncompatibleWith);
+        }
+    }
+}