Add PhotoUpload feature for uploading photos to file storage

Adds UploadPhotoCommand/Handler/Validator and PhotoUploadRepository, invoked via MediatR by other features' commands (e.g. a future CreateBreweryPhoto) rather than exposed through its own controller. Wires the feature and IFileStorageProvider into API.Core.

web/backend/Features/Features.PhotoUpload/Commands/UploadPhoto/UploadPhotoCommand.cs

@@ -0,0 +1,11 @@
+using MediatR;
+using Microsoft.AspNetCore.Http;
+
+namespace Features.ImageUploads.Commands.UploadPhoto;
+
+/// <summary>
+///     Uploads a new photo to storage under <c>{Key}/{generated photo id}</c> and records it. Sent by
+///     other features' handlers (e.g. a brewery photo upload command) rather than bound from a request
+///     directly, since this feature exposes no controller of its own.
+/// </summary>
+public record UploadPhotoCommand(Guid UploadedById, string Key, IFormFile File) : IRequest<Guid>;