Generalizes the historical run_id linkage so a key date can point at a
contract-analysis run or a formal contract entity.
source_idstring uuid nullable
status'proposed' | 'active' | 'superseded'
Lifecycle of a key date.
Distinct from the AnnualKeyDates.active boolean, which only toggles
notifications on an already-real key date. An ACTIVE-status key date may
still have active=False notifications.