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/Core/Musify.Application/Genres/GetAvailableGenres.cs

@@ -0,0 +1,26 @@
+using Mediator;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Genres
+{
+    public record GetAvailableGenresQuery : IQuery<IReadOnlyList<AvailableGenreResponse>>;
+
+    public record AvailableGenreResponse(Genre Genre, IReadOnlyList<Genre> IncompatibleWith);
+
+    public class GetAvailableGenresQueryHandler
+        : IQueryHandler<GetAvailableGenresQuery, IReadOnlyList<AvailableGenreResponse>>
+    {
+        public ValueTask<IReadOnlyList<AvailableGenreResponse>> Handle(GetAvailableGenresQuery request, CancellationToken cancellationToken)
+        {
+            var all = Enum.GetValues<Genre>();
+
+            IReadOnlyList<AvailableGenreResponse> result = all
+                .Select(genre => new AvailableGenreResponse(
+                    genre,
+                    all.Where(other => !GenreCompatibility.AreCompatible(genre, other)).ToList()))
+                .ToList();
+
+            return ValueTask.FromResult(result);
+        }
+    }
+}

backend/Core/Musify.Application/Genres/GetGenres.cs

@@ -0,0 +1,31 @@
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Genres
+{
+    public record GetGenresQuery : IQuery<IReadOnlyList<GenreResponse>>;
+
+    public record GenreResponse(Genre Genre, int TrackCount);
+
+    public class GetGenresQueryHandler(IDatabase database)
+        : IQueryHandler<GetGenresQuery, IReadOnlyList<GenreResponse>>
+    {
+        public async ValueTask<IReadOnlyList<GenreResponse>> Handle(GetGenresQuery request, CancellationToken cancellationToken)
+        {
+            var rows = await database.TrackTags
+                .AsNoTracking()
+                .Where(tag => database.UserHasTracks.Any(userTrack => userTrack.TrackId == tag.TrackId))
+                .GroupBy(tag => tag.Tag)
+                .Select(group => new { Genre = group.Key, TrackCount = group.Select(tag => tag.TrackId).Distinct().Count() })
+                .ToListAsync(cancellationToken);
+
+            return rows
+                .OrderByDescending(row => row.TrackCount)
+                .ThenBy(row => row.Genre)
+                .Select(row => new GenreResponse(row.Genre, row.TrackCount))
+                .ToList();
+        }
+    }
+}