Migration 0334, all additive. MeasurementReminder gains snoozedUntil,
lastSkippedAt and skipCount; the new MeasurementReminderEvent table is the
append-only completion ledger (kind SATISFIED|SKIPPED, occurredAt, write-time
onTime, source). CustomMetricEntry gains the deletedAt tombstone for undo
parity with every other entry surface. Consumers land in the sibling commits
of this branch.
Refs #223
