Disable libvips unfuzzed operations

[flavorjones]

Harden image processing, and add test coverage for (un)supported file types.

BMP, ICO, and PSD attachments are no longer variable, and the picture views call variant(:large) unconditionally, which raises ActiveStorage::InvariableError. Introduce Picture#large_image to fall back to the original attachment.

[Copilot]

Pull request overview

This PR hardens Active Storage image processing by restricting libvips operations/loaders and ensures the UI gracefully handles images that cannot be resized (by serving the original instead of raising on variant generation).

Changes:

• Add a Picture#large_image helper that returns a resized variant for variable images, otherwise returns the original attachment.
• Block untrusted libvips operations (and OpenSlide explicitly) and remove unsupported image types from active_storage.variable_content_types.
• Add tests covering loader selection/denial policy and the “cannot be resized” rendering path.

Tip

If you aren't ready for review, convert to a draft PR.
Click "Convert to draft" or run gh pr ready --undo.
Click "Ready for review" or run gh pr ready to reengage.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/models/picture_test.rb	Verifies large_image returns a variant for variable images and the original for non-variable types (e.g., BMP).
test/lib/vips_loader_policy_test.rb	Pins which loaders are allowed/denied under the configured libvips blocking policy.
test/controllers/leafables_controller_test.rb	Ensures a picture with a non-resizable image still renders successfully in the show view.
config/initializers/vips.rb	Blocks untrusted libvips operations and removes BMP/ICO/PSD from variable content types.
app/views/pictures/_form.html.erb	Uses large_image for the preview image to avoid invariable variant errors.
app/views/leaves/_leaf.html.erb	Uses large_image for picture leaves to avoid failing when variants can’t be generated.
app/views/leaves/_edit.html.erb	Uses large_image for the thumbnail rendering path when an image is attached.
app/views/leafables/show.html.erb	Uses large_image for the main picture rendering path (lazy-loaded).
app/models/picture.rb	Introduces large_image helper to centralize “variant-or-original” behavior.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.