Add user profile and avatar upload feature

web/backend/Features/Features.Users/Commands/UploadAvatar/UploadAvatarCommand.cs

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+using MediatR;
+using Microsoft.AspNetCore.Http;
+
+namespace Features.Auth.Commands.UploadAvatar;
+
+/// <summary>Uploads a new avatar photo for a user account, replacing any existing avatar.</summary>
+public record UploadAvatarCommand(Guid UserAccountId, IFormFile File) : IRequest<Guid>;

web/backend/Features/Features.Users/Commands/UploadAvatar/UploadAvatarHandler.cs

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+using Domain.Entities;
+using Features.Auth.Repository;
+using Features.ImageUploads.Commands.UploadPhoto;
+using MediatR;
+
+namespace Features.Auth.Commands.UploadAvatar;
+
+/// <summary>
+///     Handles <see cref="UploadAvatarCommand" />: uploads the photo via <see cref="UploadPhotoCommand" />,
+///     then saves the <see cref="UserAvatar" /> record, replacing any existing avatar for the user.
+/// </summary>
+public class UploadAvatarHandler(
+    IMediator mediator,
+    IUserAvatarRepository userAvatarRepository,
+    IUserProfileRepository userProfileRepository
+) : IRequestHandler<UploadAvatarCommand, Guid>
+{
+    public async Task<Guid> Handle(UploadAvatarCommand request, CancellationToken cancellationToken)
+    {
+        Guid photoId = await mediator.Send(
+            new UploadPhotoCommand(
+                request.UserAccountId,
+                $"avatars/{request.UserAccountId}",
+                request.File
+            ),
+            cancellationToken
+        );
+
+        Guid profileId = await userProfileRepository.GetOrCreateProfileIdAsync(
+            request.UserAccountId,
+            cancellationToken
+        );
+
+        UserAvatar avatar = new()
+        {
+            UserAvatarId = Guid.NewGuid(),
+            UserProfileId = profileId,
+            PhotoId = photoId,
+        };
+
+        await userAvatarRepository.SaveAsync(avatar, cancellationToken);
+
+        return photoId;
+    }
+}