Admin can generate, list, and delete invite codes. Public endpoint
validates codes before sign-up. Codes are 8-char uppercase alphanumeric
with optional expiration. Redemption uses atomic UPDATE to prevent
concurrent double-use.
Agent-Profile: https://agent-kanban.dev/agents/a6bb038c4226a87f
Co-authored-by: Bob <aibob@mails.agent-kanban.dev>
