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/types/activity.ts

@@ -8,6 +8,7 @@ export type ActivityEntityType =
   | "prescription"
   | "invoice"
   | "inventory"
+  | "dispense"
   | "task";
 
 export type ActivityEntry = {

backend/src/types/dispense.ts

@@ -0,0 +1,19 @@
+// The canonical Dispense shape returned by the API. Mirrors the frontend
+// `lib/dispenses.ts` Dispense type. Scoped to the active clinic; patient and
+// dispenser fields are denormalized for display.
+export type Dispense = {
+  id: string;
+  fileNumber: string;
+  name: string;
+  initials: string;
+  medication: string;
+  dose: string;
+  quantity: number;
+  unit: string;
+  prescriptionId: string | null;
+  dispensedBy: string | null;
+  dispensedByName: string;
+  dispensedAt: string; // ISO timestamp
+  notes: string | null;
+  createdAt: string;
+};

backend/src/types/prescription.ts

@@ -13,6 +13,8 @@ export type Prescription = {
   frequency: string;
   prescriber: string;
   prescribedAt: string; // YYYY-MM-DD
+  startDate: string | null; // YYYY-MM-DD, optional course start
+  endDate: string | null; // YYYY-MM-DD, optional course end (drives expiry)
   status: PrescriptionStatus;
   duration: string | null;
   notes: string | null;