feat: org-scoped prescriptions backend, wire prescriptions page

Add the prescriptions table, validation, service and CRUD routes
(/api/prescriptions, RBAC-gated; prescriber defaults to the signed-in
clinician, prescribedAt to today) and the frontend data module. The page
now loads/persists real data and computes its status KPIs.

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

backend/src/db/schema/index.ts

@@ -2,3 +2,4 @@ export * from "./auth.js";
 export * from "./patients.js";
 export * from "./notes.js";
 export * from "./appointments.js";
+export * from "./prescriptions.js";