refactor(application): co-locate each command/query with its handler

One file per use case under the feature folder (Tracks/PlayLists/Users),
dropping the Commands/Queries/Handlers sub-folders and their namespaces.
Each file now holds the record and its handler in namespace
Musify.Application.<Feature>; external usings updated accordingly.

Build: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Application/Users/Commands/CreateUserCommand.cs

@@ -1,26 +0,0 @@
-using ErrorOr;
-using Mediator;
-using Musify.Application.Users.Responses;
-using Musify.Domain.Entities;
-
-namespace Musify.Application.Users.Commands
-{
-    public record CreateUserCommand(long Id,
-        string Name,
-        string? FirstName,
-        string? SecondName)
-        : ICommand<ErrorOr<UserApplicationResponse>>
-    {
-        public static User ToEntity(CreateUserCommand command)
-        {
-            return new User
-            {
-                Id = command.Id,
-                Name = command.Name,
-                NormalizedName = command.Name.ToUpperInvariant(),
-                FirstName = command.FirstName,
-                SecondName = command.SecondName
-            };
-        }
-    }
-}

backend/Musify.Application/Users/Commands/SyncUserCommand.cs

@@ -1,13 +0,0 @@
-using ErrorOr;
-using Mediator;
-
-namespace Musify.Application.Users.Commands
-{
-    public record SyncUserCommand(
-        long Id,
-        string Name,
-        string? FirstName,
-        string? SecondName,
-        string? ProfilePictureUrl)
-        : ICommand<ErrorOr<Success>>;
-}

backend/Musify.Application/Users/CreateUser.cs

@@ -3,11 +3,30 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Musify.Application.Contracts;
-using Musify.Application.Users.Commands;
 using Musify.Application.Users.Responses;
+using Musify.Domain.Entities;
 
-namespace Musify.Application.Users.Handlers
+namespace Musify.Application.Users
 {
+    public record CreateUserCommand(long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName)
+        : ICommand<ErrorOr<UserApplicationResponse>>
+    {
+        public static User ToEntity(CreateUserCommand command)
+        {
+            return new User
+            {
+                Id = command.Id,
+                Name = command.Name,
+                NormalizedName = command.Name.ToUpperInvariant(),
+                FirstName = command.FirstName,
+                SecondName = command.SecondName
+            };
+        }
+    }
+
     public class CreateUserCommandHandler(IDatabase database, ILogger<CreateUserCommandHandler> logger)
         : ICommandHandler<CreateUserCommand, ErrorOr<UserApplicationResponse> >
     {
@@ -37,4 +56,4 @@ namespace Musify.Application.Users.Handlers
             return UserApplicationResponse.FromEntity(newUser);
         }
     }
-}
+}

backend/Musify.Application/Users/GetUserById.cs

@@ -2,11 +2,13 @@ using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
-using Musify.Application.Users.Queries;
 using Musify.Application.Users.Responses;
 
-namespace Musify.Application.Users.Handlers
+namespace Musify.Application.Users
 {
+    public record GetUserByIdQuery(long Id)
+        : IQuery<ErrorOr<UserApplicationResponse> >;
+
     public class GetUserByIdQueryHandler(IDatabase database)
         : IQueryHandler<GetUserByIdQuery, ErrorOr<UserApplicationResponse> >
     {
@@ -24,4 +26,4 @@ namespace Musify.Application.Users.Handlers
             return user;
         }
     }
-}
+}

backend/Musify.Application/Users/GetUsers.cs

@@ -2,13 +2,15 @@ using ErrorOr;
 using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Musify.Application.Pagination;
-using Musify.Application.Users.Queries;
 using Musify.Application.Users.Responses;
 using X.PagedList.EF;
 using Musify.Application.Contracts;
 
-namespace Musify.Application.Users.Handlers
+namespace Musify.Application.Users
 {
+    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch = null)
+        : IQuery<ErrorOr<PaginatedResponse<UserApplicationResponse> >>;
+
     public class GetUsersQueryHandler(IDatabase database)
         : IQueryHandler<GetUsersQuery, ErrorOr<PaginatedResponse<UserApplicationResponse> > >
     {
@@ -32,4 +34,4 @@ namespace Musify.Application.Users.Handlers
             return PaginatedResponse<UserApplicationResponse>.FromPagedList(pagedUser);
         }
     }
-}
+}

backend/Musify.Application/Users/Queries/GetUserByIdQuery.cs

@@ -1,9 +0,0 @@
-using ErrorOr;
-using Mediator;
-using Musify.Application.Users.Responses;
-
-namespace Musify.Application.Users.Queries
-{
-    public record GetUserByIdQuery(long Id)
-        : IQuery<ErrorOr<UserApplicationResponse> >;
-}

backend/Musify.Application/Users/Queries/GetUsersQuery.cs

@@ -1,10 +0,0 @@
-using ErrorOr;
-using Mediator;
-using Musify.Application.Pagination;
-using Musify.Application.Users.Responses;
-
-namespace Musify.Application.Users.Queries
-{
-    public record GetUsersQuery(int PageNumber, int PageSize, string? UsernameSearch = null)
-        : IQuery<ErrorOr<PaginatedResponse<UserApplicationResponse> >>;
-}

backend/Musify.Application/Users/SyncUser.cs

@@ -3,11 +3,18 @@ using Mediator;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Musify.Application.Contracts;
-using Musify.Application.Users.Commands;
 using Musify.Domain.Entities;
 
-namespace Musify.Application.Users.Handlers
+namespace Musify.Application.Users
 {
+    public record SyncUserCommand(
+        long Id,
+        string Name,
+        string? FirstName,
+        string? SecondName,
+        string? ProfilePictureUrl)
+        : ICommand<ErrorOr<Success>>;
+
     public class SyncUserCommandHandler(IDatabase database, ILogger<SyncUserCommandHandler> logger)
         : ICommandHandler<SyncUserCommand, ErrorOr<Success>>
     {