perf(worker): Decode image once per file in ImageSharpProcessor #153

Problem

ImageProxyMiddleware reads the source file into a byte[] once (good), but then each of the three generators (ThumbnailGenerator, FullPreviewGenerator, BlurhashGenerator) independently calls Image.LoadAsync from a fresh MemoryStream. This means a large image (e.g. a 50 MP HEIC or RAW file) is fully decoded three times per ingestion.

Proposed solution

Load Image<Rgba32> once in ImageProxyMiddleware (or in ImageSharpProcessor) and pass the already-decoded image to each generator. The simplest approach is to add an overload / new interface method that accepts a pre-loaded image:

internal interface IImageProcessor
{
    Task<byte[]> CreateThumbnailAsync(Image source, CancellationToken ct);
    Task<byte[]> CreateFullPreviewAsync(Image source, CancellationToken ct);
    Task<string> ComputeBlurhashAsync(Image<Rgba32> source, CancellationToken ct);
}

ImageProxyMiddleware.InvokeAsync would load the image once:

using var image = await Image.LoadAsync<Rgba32>(sourceStream, ct);
var proxyFiles = await Task.WhenAll(generators.Select(g => WriteProxyAsync(g, image)));

The IImageProxyGenerator.GenerateAsync signature would need to change from Stream to Image (or a wrapper type), and the Stream-based overloads can be removed since the middleware now owns the decode lifetime.

Impact

Reduces decode work from O(n × generators) to O(n) per file. Most noticeable for high-resolution sources (HEIC, TIFF, uncompressed RAW) where decoding dominates processing time.