feat: complete remaining Go backend modules

tests/billing-http-contract.test.ts

@@ -0,0 +1,17 @@
+import { readFile } from "node:fs/promises";
+import { describe, expect, it } from "vitest";
+
+describe("billing HTTP compatibility fixture", () => {
+  it("freezes routes, refreshed-session scope, money, and admin mutation rules", async () => {
+    const fixture = JSON.parse(await readFile("contracts/billing/http-v1.json", "utf8"));
+    expect(fixture).toMatchObject({
+      version: 1, currency: "CNY", moneyUnit: "fen",
+      statuses: { insufficientBalance: 402, idempotencyConflict: 409, unboundOrganization: 422, infrastructure: 500 },
+      pricePatch: { minimum: 1, maximum: 1000, precision: 4, tierPairRequired: true }
+    });
+    expect(fixture.routes).toEqual(expect.arrayContaining([
+      expect.objectContaining({ method: "GET", path: "/api/billing", organizationSource: "refreshed_session" }),
+      expect.objectContaining({ method: "PATCH", path: "/api/admin/billing/prices/{id}", requirement: "super_admin" })
+    ]));
+  });
+});