Add self-service account management commands (delete, update username/email/password/profile)

web/backend/Features/Features.Auth/Commands/DeleteAccount/DeleteAccountCommand.cs

@@ -0,0 +1,10 @@
+using MediatR;
+
+namespace Features.Auth.Commands.DeleteAccount;
+
+/// <summary>Permanently deletes the given user account.</summary>
+/// <param name="UserAccountId">
+///     The authenticated caller's own ID, extracted from the access token -- never bind this from
+///     client-supplied input.
+/// </param>
+public record DeleteAccountCommand(Guid UserAccountId) : IRequest;

web/backend/Features/Features.Auth/Commands/DeleteAccount/DeleteAccountHandler.cs

@@ -0,0 +1,33 @@
+using Domain.Exceptions;
+using Features.Auth.Identity;
+using MediatR;
+using Microsoft.AspNetCore.Identity;
+
+namespace Features.Auth.Commands.DeleteAccount;
+
+/// <summary>Handles <see cref="DeleteAccountCommand" /> by deleting the user via <see cref="UserManager{TUser}" />.</summary>
+public class DeleteAccountHandler(UserManager<ApplicationUser> userManager)
+    : IRequestHandler<DeleteAccountCommand>
+{
+    /// <exception cref="NotFoundException">Thrown when the account does not exist.</exception>
+    /// <exception cref="ConflictException">
+    ///     Thrown when the account cannot be deleted because other records (posts, comments, photos,
+    ///     follows) still reference it.
+    /// </exception>
+    public async Task Handle(DeleteAccountCommand request, CancellationToken cancellationToken)
+    {
+        ApplicationUser user =
+            await userManager.FindByIdAsync(request.UserAccountId.ToString())
+            ?? throw new NotFoundException("User account not found");
+
+        IdentityResult result = await userManager.DeleteAsync(user);
+        if (!result.Succeeded)
+        {
+            if (result.Errors.Any(e => e.Code == "AccountHasDependentRecords"))
+                throw new ConflictException(
+                    "Account cannot be deleted while it still has associated posts, comments, photos, or follows."
+                );
+            throw new ConflictException(string.Join("; ", result.Errors.Select(e => e.Description)));
+        }
+    }
+}