fix: stop leaking a temp file on every image send

[ThiagoBauken]

What & why

SendImage created a temp file (os.CreateTemp) to hold the resized thumbnail, read it back, and only deferred Close() — never Remove(). So every image send left a file behind in TMPDIR, eventually exhausting disk on busy servers.

Change

The thumbnail is now resized and JPEG-encoded entirely in memory via a small jpegThumbnail helper — no temp file, and two fewer filesystem operations. Behavior is otherwise identical.

Testing

• TestJpegThumbnail — returns a decodable JPEG bounded by the requested size; fails on the un-resized path (red→green).
• go build ./... · go vet ./... · go test ./... — pass on host.
• GOOS=linux GOARCH=amd64 go build . + go test ./... / go test -race ./... — pass on linux/amd64 (Docker golang:1.25).

No API change — internal fix.

[gemini-code-assist]

Code Review

This pull request refactors the thumbnail generation in SendImage by introducing an in-memory helper function jpegThumbnail and an accompanying unit test, successfully eliminating the use of temporary files and preventing potential file leaks. The review feedback correctly points out that the img parameter in jpegThumbnail could be nil, which would lead to a runtime panic, and suggests adding a defensive nil check.

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[gemini-code-assist]

The img parameter is of interface type image.Image, which can be nil. If a nil image is passed to resize.Thumbnail, it will cause a runtime panic because resize.Thumbnail attempts to access the image's bounds. To ensure robust and defensive programming, add a nil check at the beginning of the function.

func jpegThumbnail(img image.Image, width, height uint) ([]byte, error) {
	if img == nil {
		return nil, errors.New("cannot create thumbnail from nil image")
	}
	thumb := resize.Thumbnail(width, height, img, resize.Lanczos3)
	var buf bytes.Buffer
	if err := jpeg.Encode(&buf, thumb, nil); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

[ThiagoBauken]

Thanks — addressed in 7d57cfd: jpegThumbnail now returns an error on a nil image before it ever reaches resize.Thumbnail, and TestJpegThumbnailNil covers it. (c3ade4f additionally proves, on a real 1536x1024 image, that the in-memory output is byte-identical to the previous temp-file path.)

[ThiagoBauken]

Thanks — added the nil guard. jpegThumbnail now returns an error for a nil image instead of letting resize.Thumbnail dereference its bounds and panic. Covered by TestJpegThumbnailNil, which fails (panics) without the guard and passes with it. go build/vet/test pass on host and it cross-compiles for linux/amd64.