Add PostgreSQL-backed background worker

apps/worker/tests/recurring-invoices.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, test } from "bun:test";
+
+import { nextDueDate } from "../../web/src/server/services/recurring-invoices";
+
+describe("recurring invoice scheduling", () => {
+  test("advances weekly schedules from their scheduled occurrence", () => {
+    const scheduledFor = new Date("2026-08-17T12:00:00.000Z");
+    expect(nextDueDate("weekly", scheduledFor).toISOString()).toBe(
+      "2026-08-24T12:00:00.000Z",
+    );
+  });
+
+  test("does not mutate the source date", () => {
+    const scheduledFor = new Date("2026-08-17T12:00:00.000Z");
+    nextDueDate("monthly", scheduledFor);
+    expect(scheduledFor.toISOString()).toBe("2026-08-17T12:00:00.000Z");
+  });
+});