Transcode an uploaded image to WebP and store one file per target.

The source bytes (of the validated mime) are decoded once, then each target is downscaled to its max width and encoded to WebP at its quality; each variant is encrypted and uploaded under a fresh .webp filename. Returns the stored filenames in the same order as targets — so a caller wanting a full-size image plus a thumbnail passes both targets and destructures the two filenames back out.

The image pipeline (~1MB of codec wasm) is dynamically imported here so it is loaded only on the first upload, never at cold boot.