0.20.1: overlapping posts no longer 500

Fixes

• Two identical job posts in flight at once no longer answer 500. Both computed the same slug, and the one that lost the race hit the slug's unique index. It now retries with a fresh slug, and a retry carrying the same Idempotency-Key returns the original listing (#186).
• Text cleanup drops lone surrogates, and the length cap never cuts an emoji in half (#185, thanks @maegminhui-tech).
• Landing policies ignore inherited object keys (#184).