Recognize .webp as a valid scaled-derivative extension

[jdub233]

The lambda's request-routing logic in getOrCreateObject.js filters scaled-derivative requests by extension whitelist:

const sizeMatch = pathname.match(/-(\d+)x(\d+)\.(jpg|jpeg|png|gif)$/);

.webp is not included. Requests for image-300x200.webp don't match sizeMatch, fall through the "is this a render request" branch, and 404. The Sharp invocation itself is format-agnostic (no explicit .toFormat() is set, so output format follows input), so once routing matches .webp, end-to-end rendering works without further changes.

This would be an enabling change for the efforts around the Questrom site migration. That site's WordPress database already carries correct .webp size-suffix references, so this regex fix unblocks rendering for those assets.

Goals

• Add webp to the sizeMatch regex in getOrCreateObject.js
• Add webp to the originalPath reconstruction regex in the same file's NoSuchKey branch
• Delete the unused getOriginalS3Key() function in resizeAndSave.js (carries a stale narrower regex (jpg|png); the path math is now done in getOrCreateObject.js)
• Add a WebP fixture to the test suite (sequence this first for test-driven development)

Acceptance criteria

• A request for a WebP scaled derivative whose original exists at original_media/... generates and serves the resized .webp correctly
• Existing jpg/jpeg/png/gif behavior unchanged
• WebP references in the Questrom WP database resolve end-to-end against a dev deployment

[jdub233]

Fixed by #68