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/API/API.Core/Controllers/UserController.cs

@@ -1,43 +0,0 @@
-using Domain.Entities;
-using Microsoft.AspNetCore.Mvc;
-using Service.UserManagement.User;
-
-namespace API.Core.Controllers
-{
-    /// <summary>
-    /// Provides read-only endpoints for retrieving user accounts.
-    /// </summary>
-    /// <param name="userService">Service used to query user account data.</param>
-    [ApiController]
-    [Route("api/[controller]")]
-    public class UserController(IUserService userService) : ControllerBase
-    {
-        /// <summary>
-        /// Retrieves a paginated list of user accounts.
-        /// </summary>
-        /// <param name="limit">The maximum number of user accounts to return, or <c>null</c> for no limit.</param>
-        /// <param name="offset">The number of user accounts to skip before returning results, or <c>null</c> for no offset.</param>
-        /// <returns>A <c>200 OK</c> result containing the collection of <see cref="UserAccount"/> entities.</returns>
-        [HttpGet]
-        public async Task<ActionResult<IEnumerable<UserAccount>>> GetAll(
-            [FromQuery] int? limit,
-            [FromQuery] int? offset
-        )
-        {
-            var users = await userService.GetAllAsync(limit, offset);
-            return Ok(users);
-        }
-
-        /// <summary>
-        /// Retrieves a single user account by its unique identifier.
-        /// </summary>
-        /// <param name="id">The unique identifier of the user account to retrieve.</param>
-        /// <returns>A <c>200 OK</c> result containing the matching <see cref="UserAccount"/>.</returns>
-        [HttpGet("{id:guid}")]
-        public async Task<ActionResult<UserAccount>> GetById(Guid id)
-        {
-            var user = await userService.GetByIdAsync(id);
-            return Ok(user);
-        }
-    }
-}