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

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+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>;

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

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+using Domain.Entities;
+using Features.ImageUploads.Repository;
+using Infrastructure.FileUpload;
+using MediatR;
+
+namespace Features.ImageUploads.Commands.UploadPhoto;
+
+/// <summary>Handles <see cref="UploadPhotoCommand" />.</summary>
+public class UploadPhotoHandler(IFileStorageProvider fileStorageProvider, IPhotoUploadRepository photoUploadRepository)
+    : IRequestHandler<UploadPhotoCommand, Guid>
+{
+    public async Task<Guid> Handle(UploadPhotoCommand request, CancellationToken cancellationToken)
+    {
+        Guid photoId = Guid.NewGuid();
+        string key = $"{request.Key}/{photoId}";
+
+        await using Stream stream = request.File.OpenReadStream();
+
+
+        await fileStorageProvider.UploadAsync(
+            key,
+            stream,
+            request.File.ContentType,
+            cancellationToken
+        );
+
+
+        Photo photo = new()
+        {
+            Hyperlink = key,
+            UploadedById = request.UploadedById,
+        };
+
+
+        await photoUploadRepository.CreateAsync(photo,cancellationToken);
+
+
+        return photoId;
+    }
+}

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

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+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.");
+    }
+}