feat: invoices — patient billing with installments + PDF export

Backend (new `invoice` RBAC resource, granted to clinicians + reception):
- invoices table (line items + installments as JSONB), types, zod validation,
  service (CRUD + splitIntoInstallments + auto invoice numbers), org-scoped
  REST routes mounted at /api/invoices, activity logging (migration 0015)

Frontend:
- lib/invoices.ts API client + money/date helpers
- /invoices page: list with KPIs and search, create/edit dialog (searchable
  patient combobox, inline line-item editor, live total), detail sheet to split
  a bill into equal monthly installments, delete, and Download PDF
- dependency-free PDF via a print-styled window (browser "Save as PDF")
- sidebar "Invoices" entry under the Patients group; "Added by AI" badge honored

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

frontend/app/(app)/invoices/page.tsx

@@ -0,0 +1,10 @@
+import { InvoicesView } from "@/components/invoices/invoices-view";
+import { SidebarInset } from "@/components/ui/sidebar";
+
+export default function InvoicesPage() {
+  return (
+    <SidebarInset className="flex flex-1 flex-col overflow-y-auto">
+      <InvoicesView />
+    </SidebarInset>
+  );
+}