generate_base_drafts hardens every base draft to a transparent PNG with
_harden_transparency. When the provider returned a non-PNG file (webp,
jpg, or gif), the hardened PNG is saved under a new path and the original
draft is left in cache/images. Nothing prunes that directory outside the
gateway housekeeping loop, so a CLI or desktop draft round leaks one
original per non-PNG draft.
Remove the original after a successful hardening when the output path
differs from the input. PNG inputs (including mixed-case suffixes like
.PNG) are hardened in place so a case-insensitive filesystem cannot
treat with_suffix(".png") as a different file and unlink the output.
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