Delete Puzzle Endpoint

Remove a puzzle. Site admins only.

SOFT, and not primarily for recoverability: a site-wide puzzle's slug is a claim in the global handle namespace, so dropping the row would free the name for someone else and a link that used to be a puzzle would become a member's profile. The slug stays taken. Attempts are left intact, so restoring returns the leaderboard exactly as it was.

Audited in the admin action log against the puzzle's author.