Add global crop_fill percent (default 90, 1-100) as a settings key with
FastAPI 4xx validation. Wires the value through to the inventory PNG
encode (non-retro: stored PNGs untouched). Adds a throwaway
TapePrototype.svelte behind ?tape-prototype for the #99 design sandbox.
Gates: check_requirements PASS, check_coverage PASS, settings tests 8/8.
Owner-directed, LAN/HTTP dev-test only. When the env flag is set,
GET /api/v1/settings returns decrypted secret values (string) instead of
{configured: bool}; the UI prefills and shows them in clear text. Default
off -> SETTINGS-75 masking and the existing tests stay intact. Revert
before any non-UAT deployment.