- VaccinationRecord holds one administered dose: date, catalogue slug or
the Pass's own wording, dose and series numbers, batch code, site, and
optional links to a practitioner, a visit and a booster reminder
- VaccinationDocumentLink keeps the scanned Impfpass page beside the dose
- MeasurementReminder gains a nullable vaccination_antigen match key so a
logged dose can clear a booster without a second reminder engine
- migration 0330, additive apart from that one nullable column
