Migrate UserManagement to a vertical slice (Features.UserManagement)

Same pattern as the Breweries migration: Service.UserManagement and the
UserAccount repository fold into Features.UserManagement, with MediatR
queries replacing UserService. UpdateUserCommand/Handler carry forward
IUserService.UpdateAsync as-is (it has no HTTP route today, and adding one
is a separate decision from this migration). Adds handler/repository test
coverage that didn't exist before, since Service.UserManagement had no
test project.

web/backend/Features/Features.UserManagement/Queries/GetUserById/GetUserByIdHandler.cs

@@ -0,0 +1,23 @@
+using Domain.Entities;
+using Domain.Exceptions;
+using Features.UserManagement.Repository;
+using MediatR;
+
+namespace Features.UserManagement.Queries.GetUserById;
+
+/// <summary>
+/// Handles <see cref="GetUserByIdQuery"/> by looking up the matching user account.
+/// </summary>
+/// <param name="repository">Repository used to query user account data.</param>
+public class GetUserByIdHandler(IUserAccountRepository repository)
+    : IRequestHandler<GetUserByIdQuery, UserAccount>
+{
+    /// <exception cref="NotFoundException">Thrown when no user account exists with the given ID.</exception>
+    public async Task<UserAccount> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+    {
+        var user = await repository.GetByIdAsync(request.UserAccountId);
+        if (user is null)
+            throw new NotFoundException($"User with ID {request.UserAccountId} not found");
+        return user;
+    }
+}

web/backend/Features/Features.UserManagement/Queries/GetUserById/GetUserByIdQuery.cs

@@ -0,0 +1,9 @@
+using Domain.Entities;
+using MediatR;
+
+namespace Features.UserManagement.Queries.GetUserById;
+
+/// <summary>
+/// Retrieves a single user account by its unique identifier.
+/// </summary>
+public record GetUserByIdQuery(Guid UserAccountId) : IRequest<UserAccount>;