feat(core): add claim command persistence layer (#46)

persistClaimSubmit and persistClaimApprove, following the listing.upsert
pattern. Shared transaction plumbing extracted to persist/shared.ts.
Multi-event commands stamp idempotency_key on the first outbox row only,
matching the (tenant_id, idempotency_key) unique index.

Approve locks the claim and listing rows and refuses to move ownership away
from a sitting owner, enforcing spec section 9.4 in the store rather than
only in the pure handler.

Claimant user upsert on (tenant_id, email) is provisional pending #44.

Refs #44, #45

Signed-off-by: Matthew Wren <info@origindev.com>

Author: OriginDevIT