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/UploadPhotoValidator.cs

@@ -0,0 +1,33 @@
+using FluentValidation;
+
+namespace Features.ImageUploads.Commands.UploadPhoto;
+
+/// <summary>Validates <see cref="UploadPhotoCommand" />.</summary>
+public class UploadPhotoValidator : AbstractValidator<UploadPhotoCommand>
+{
+    public UploadPhotoValidator()
+    {
+        RuleFor(x => x.UploadedById)
+            .NotEmpty()
+            .WithMessage("UploadedById is required.");
+
+        RuleFor(x => x.File)
+            .NotNull()
+            .WithMessage("File is required.");
+
+        RuleFor(x => x.Key)
+            .NotEmpty()
+            .WithMessage("Key is required.");
+
+
+        RuleFor(x => x.File.Length)
+            .GreaterThan(0)
+            .When(x => x.File is not null)
+            .WithMessage("File cannot be empty.");
+
+        RuleFor(x => x.File.ContentType)
+            .Must(contentType => contentType.StartsWith("image/", StringComparison.OrdinalIgnoreCase))
+            .When(x => x.File is not null)
+            .WithMessage("File must be an image.");
+    }
+}