JPG utils for smartstacks

[timbeccue]

This PR adds two capabilities: generate smartstack jpg filenames, and create stretched jpg files.

Jpg stretching uses auto-stretch (see this repository) and pillow for file creation. Both are new dependencies for the project.

Jpg filenames are created using a simple modification of the smartstack filename, inserting large_thumbnail or small_thumbnail before the file extension.

[cmccully]

Looks fine.

[Copilot]

Pull request overview

This PR introduces JPEG-related utilities to support smartstack workflows: generating deterministic thumbnail JPEG filenames from smartstack FITS names, and producing display-ready stretched grayscale JPEG previews using auto-stretch + Pillow.

Changes:

• Add banzai.utils.jpg_utils with FITS-to-display stretching and JPEG writing helpers.
• Add make_jpg_filenames() to derive small/large thumbnail JPEG filenames from smartstack filenames.
• Add test coverage for the new utilities and introduce new runtime dependencies (auto-stretch, pillow).

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
uv.lock	Locks new dependencies (auto-stretch, pillow) for reproducible installs.
pyproject.toml	Adds auto-stretch and pillow to project dependencies.
banzai/utils/jpg_utils.py	Implements stretching/decimation and grayscale JPEG saving helpers.
banzai/utils/file_utils.py	Adds smartstack-derived thumbnail JPEG filename generator.
banzai/tests/test_jpg_utils.py	Tests stretch behavior (dtype/shape/NaN handling) and JPEG sizing/mode.
banzai/tests/test_file_utils.py	Tests filename generation for .fits and .fits.fz inputs.

Suppressed comments (1)

banzai/utils/jpg_utils.py:51

• max_size should be validated before passing it to thumbnail(). For max_size <= 0, Pillow behavior is error-prone and the intent is clearer if we raise a ValueError up front.

    image = Image.fromarray(np.asarray(display_image, dtype=np.uint8))
    image.thumbnail((max_size, max_size))
    image.save(path, quality=quality)

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