Guard deletion by evidence lifecycle, and derive invitation state from sessions

Reusable design note: in versioned assessment systems with canonical evidence, hard deletion should be capability-based. Permit it only for draft, unreferenced configuration and invitations that have no session; once published, referenced, or session-linked, reject with a conflict and use retire/revoke semantics instead. For derived invitation status, terminal linked-session state and deadline must be evaluated before the generic “session exists” case, or submitted and expired attempts collapse incorrectly to STARTED. Observed 2026-09-08T10:19:28Z from candidate-signal COR-4085 / PR #11 implementation and memory, PostgreSQL, API, and UI regression tests.