Discard a draft (DRAFT/READY) Exchange

Soft-deletes an Exchange that has NOT yet been activated. Sets
deletedAt = NOW() so the row vanishes from every standard query
while remaining in DB for forensic access. Allowed ONLY when the
Exchange is in DRAFT or READY status (no Capability Grant
emitted under it). For ACTIVE / SUSPENDED callers MUST use
revoke instead (preserves the row as terminal-state evidence).
Terminal states (REVOKED / EXPIRED) are already non-actionable.

Optimistic locking: the version query parameter MUST match the
entity's current @Version. Mismatch → 409 Conflict.

Scope: admin.tenant.exchange.delete