backend: prescription start/end dates + dispense ledger

Add optional startDate/endDate columns to prescriptions (schema,
validation, types, service) — when set, endDate drives expiry.

Add a new append-only `dispenses` resource (schema, types, validation,
service, route at /api/dispenses) recording who received which medication,
gated on the existing inventory RBAC statement. Migration 0020.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/services/prescriptions.ts

@@ -22,6 +22,8 @@ function toPrescription(row: PrescriptionRow): Prescription {
     frequency: row.frequency,
     prescriber: row.prescriber,
     prescribedAt: row.prescribedAt,
+    startDate: row.startDate,
+    endDate: row.endDate,
     status: row.status,
     duration: row.duration,
     notes: row.notes,
@@ -45,6 +47,8 @@ function columns(orgId: string, input: PrescriptionInput, createdBy?: string) {
     duration: input.duration ?? null,
     notes: input.notes ?? null,
     source: input.source,
+    startDate: input.startDate ?? null,
+    endDate: input.endDate ?? null,
     // Only set prescribedAt when supplied; otherwise the column default (today).
     ...(input.prescribedAt ? { prescribedAt: input.prescribedAt } : {}),
     ...(createdBy ? { createdBy } : {}),