New Discount domain: DiscountRule (generic per-tenant rule with 6 types,
priority, combinable, validity window, and per-type target fields) plus its
repository. PatientSession gains applied_discount_rule_id/label audit columns
and setDiscount() now records the source rule. Migration creates the table
and columns.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
