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/db/schema/index.ts

@@ -5,6 +5,7 @@ export * from "./appointments.js";
 export * from "./prescriptions.js";
 export * from "./invoices.js";
 export * from "./inventory.js";
+export * from "./dispenses.js";
 export * from "./tasks.js";
 export * from "./activity.js";
 export * from "./messaging.js";