Integrate SeaweedFS for S3-compatible file storage and photo upload (#254)

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;
+using Microsoft.AspNetCore.Http;
+
+namespace Features.ImageUploads.Commands.UploadPhoto;
+
+/// <summary>Validates <see cref="UploadPhotoCommand" />.</summary>
+public class UploadPhotoValidator : AbstractValidator<UploadPhotoCommand>
+{
+    private static readonly byte[] PngSignature = [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
+
+    private static readonly byte[] JpegSignature = [0xFF, 0xD8, 0xFF];
+
+    private static readonly byte[] RiffSignature =
+    [
+        0x52,
+        0x49,
+        0x46,
+        0x46, // "RIFF"
+    ];
+
+    private static readonly byte[] WebPMarker =
+    [
+        0x57,
+        0x45,
+        0x42,
+        0x50, // "WEBP"
+    ];
+
+    private const long MinSizeBytes = 1024; // 1 KiB
+    private const long MaxSizeBytes = 24 * 1024 * 1024; // 24 MiB
+
+    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.ContentType)
+            .Must(contentType =>
+                contentType.StartsWith("image/", StringComparison.OrdinalIgnoreCase)
+            )
+            .WithMessage("File must be an image.");
+
+        RuleFor(x => x.File.Length)
+            .InclusiveBetween(MinSizeBytes, MaxSizeBytes)
+            .WithMessage("The file must be between 1 KiB and 24 MiB.");
+
+        RuleFor(x => x.File)
+            .MustAsync(HaveValidImageSignatureAsync)
+            .WithMessage("The file is not a valid PNG, JPEG, or WebP image.");
+    }
+
+    private static async Task<bool> HaveValidImageSignatureAsync(
+        IFormFile file,
+        CancellationToken cancellationToken
+    )
+    {
+        var buffer = new byte[12];
+        await using var stream = file.OpenReadStream();
+        var bytesRead = await stream.ReadAsync(
+            buffer.AsMemory(0, buffer.Length),
+            cancellationToken
+        );
+
+        return IsPng(buffer, bytesRead) || IsJpeg(buffer, bytesRead) || IsWebP(buffer, bytesRead);
+    }
+
+    private static bool IsPng(byte[] buffer, int bytesRead) =>
+        bytesRead >= PngSignature.Length
+        && buffer.AsSpan(0, PngSignature.Length).SequenceEqual(PngSignature);
+
+    private static bool IsJpeg(byte[] buffer, int bytesRead) =>
+        bytesRead >= JpegSignature.Length
+        && buffer.AsSpan(0, JpegSignature.Length).SequenceEqual(JpegSignature);
+
+    private static bool IsWebP(byte[] buffer, int bytesRead) =>
+        bytesRead >= 12
+        && buffer.AsSpan(0, 4).SequenceEqual(RiffSignature)
+        && buffer.AsSpan(8, 4).SequenceEqual(WebPMarker);
+}