refactor(persistence): migrate fulfillment-store to canonical atomic-write util (DC-103) [glm-grade=A]

writeState() drops its private tmp+writeFileSync+rename copy (no fsync,
Date.now() tmp names, best-effort chmod) and delegates to
src/utils/atomic-write.js atomicWriteJSON (exclusive-create tmp, fsync,
rename, parent-dir fsync, 0600). A torn stripe-fulfillments.json could
previously make a webhook retry mint a SECOND valid license key for an
order that already has one. Both consumers (routes/billing.js,
scripts/stripe-license-bridge.js) JSON.parse only - trailing-newline
drop in the canonical serializer is harmless. Unused crypto require
removed. +2 DC-103 pins: full lifecycle 0600/complete/zero-leftovers,
and dual-instance interleaved writes (bridge+API file-IPC) with no tmp
collisions. 121 suites / 2776 tests green.

Judge: GLM-5.3 round-1 A (deleg_bd49a97b), URN urn:ump:layk7h326sqymcsh6tiusrs2sdbqdcx6ygvcuwwoxqn4rf7ycoua

dashcaddy-api/__tests__/atomic-write-dc099.test.js

@@ -360,3 +360,65 @@ describe('DC-102 share-store on canonical atomic-write (real fs)', () => {
     expect(leftovers).toEqual([]);
   });
 });
+
+// DC-103: fulfillment-store (Stripe license state, shared file-IPC between
+// the API's lookup endpoint and the stripe-license-bridge process) migrated
+// off its private tmp+rename copy onto the canonical writer. Pins: the file
+// lands at 0600, parses as complete JSON after every mutation class, and no
+// temp files survive — a torn write here would make a webhook retry mint a
+// SECOND valid license key for an order that already has one.
+describe('DC-103 fulfillment-store on canonical atomic-write (real fs)', () => {
+  let dir, store;
+
+  beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'dc103-fulfill-'));
+    store = require('../src/billing/fulfillment-store').createFulfillmentStore({ filePath: path.join(dir, 'stripe-fulfillments.json') });
+  });
+  afterEach(() => { try { fs.rmSync(dir, { recursive: true, force: true }); } catch (_) {} });
+
+  test('claim → saveLicense → claimDelivery → markDelivered lands at 0600, complete JSON, no temp leftovers', async () => {
+    const claimed = await store.claim({ eventId: 'evt_dc103', sessionId: 'cs_dc103', productId: 'pro-30d', durationDays: 30, email: 'dc103@x.com' });
+    expect(claimed.claimed).toBe(true);
+    const saved = await store.saveLicense({ eventId: 'evt_dc103', sessionId: 'cs_dc103', code: 'DC103-KEY-XXXX', codeId: 'kg_dc103' });
+    expect(saved.saved).toBe(true);
+    const delivery = await store.claimDelivery({ sessionId: 'cs_dc103', ownerToken: 'own_1' });
+    expect(delivery.claimed).toBe(true);
+    const delivered = await store.markDelivered({ sessionId: 'cs_dc103', ownerToken: 'own_1', deliveredVia: 'smtp' });
+    expect(delivered.saved).toBe(true);
+
+    const file = path.join(dir, 'stripe-fulfillments.json');
+    const st = fs.statSync(file);
+    expect(st.mode & 0o777).toBe(0o600);
+
+    // complete JSON carrying the full lifecycle — a torn write would fail
+    // JSON.parse right here
+    const data = JSON.parse(fs.readFileSync(file, 'utf8'));
+    expect(data.bySessionId['cs_dc103'].status).toBe('delivered');
+    expect(data.bySessionId['cs_dc103'].code).toBe('DC103-KEY-XXXX');
+    expect(data.bySessionId['cs_dc103'].eventId).toBe('evt_dc103');
+    // both index maps point at the same record
+    expect(data.byEventId['evt_dc103'].sessionId).toBe('cs_dc103');
+
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'stripe-fulfillments.json');
+    expect(leftovers).toEqual([]);
+  });
+
+  test('back-to-back mutations across separate store instances never collide on tmp names', async () => {
+    // Two processes share this file (bridge + API lookup). Two store
+    // instances writing interleaved must never collide on the same tmp name
+    // (the counter is per-process, so cross-instance is the real pin).
+    const storeA = require('../src/billing/fulfillment-store').createFulfillmentStore({ filePath: path.join(dir, 'stripe-fulfillments.json') });
+    const storeB = require('../src/billing/fulfillment-store').createFulfillmentStore({ filePath: path.join(dir, 'stripe-fulfillments.json') });
+    for (let i = 0; i < 6; i += 1) {
+      const a = await storeA.claim({ eventId: `evt_a${i}`, sessionId: `cs_a${i}`, productId: 'pro-30d', durationDays: 30, email: 'a@x.com' });
+      expect(a.claimed).toBe(true);
+      const b = await storeB.claim({ eventId: `evt_b${i}`, sessionId: `cs_b${i}`, productId: 'pro-30d', durationDays: 30, email: 'b@x.com' });
+      expect(b.claimed).toBe(true);
+    }
+    const file = path.join(dir, 'stripe-fulfillments.json');
+    const data = JSON.parse(fs.readFileSync(file, 'utf8'));
+    expect(Object.keys(data.byEventId)).toHaveLength(12);
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'stripe-fulfillments.json');
+    expect(leftovers).toEqual([]);
+  });
+});