Unify API route parameters, return IResult everywhere, merge picture upload DTOs and centralize validation limits

Guid and long route constraints, id/trackId parameter names, Update*Request without the New prefix,
sentence-case operation summaries, shared security scheme id and a single Limits class.

backend/Hosts/Musify.Api/Endpoints/GenreEndpoints.cs

@@ -13,28 +13,30 @@ public static class GenreEndpoints
 
         group.MapGet("/", GetGenres)
             .WithName("GetGenres")
-            .WithSummary("Get The Genres That Have At Least One Track, Most Populated First. Works Anonymously.")
+            .WithSummary("Get the genres that have at least one track, most populated first. Works anonymously")
             .Produces<IReadOnlyList<GenreResponse>>();
 
         group.MapGet("/available", GetAvailableGenres)
             .WithName("GetAvailableGenres")
-            .WithSummary("Get Every Genre A Track Can Be Tagged With, Including The Genres It Cannot Be Combined With. Works Anonymously.")
+            .WithSummary("Get every genre a track can be tagged with, including the genres it cannot be combined with. Works anonymously")
             .Produces<IReadOnlyList<AvailableGenreResponse>>();
 
         return app;
     }
 
-    private static async Task<IReadOnlyList<AvailableGenreResponse>> GetAvailableGenres(
+    private static async Task<IResult> GetAvailableGenres(
         IMediator mediator,
         CancellationToken cancellationToken)
     {
-        return await mediator.Send(new GetAvailableGenresQuery(), cancellationToken);
+        var result = await mediator.Send(new GetAvailableGenresQuery(), cancellationToken);
+        return Results.Ok(result);
     }
 
-    private static async Task<IReadOnlyList<GenreResponse>> GetGenres(
+    private static async Task<IResult> GetGenres(
         IMediator mediator,
         CancellationToken cancellationToken)
     {
-        return await mediator.Send(new GetGenresQuery(), cancellationToken);
+        var result = await mediator.Send(new GetGenresQuery(), cancellationToken);
+        return Results.Ok(result);
     }
 }