refactor(persistence): migrate invite-store to canonical atomic-write util (DC-100) [glm-grade=A]

Drops invite-store's private _atomicWriteJSON copy (pid+Date.now() tmp
names, no fsync, no failure cleanup) in favor of src/utils/atomic-write.js:
fsync'd same-dir tmp+rename, exclusive-create 0600, parent-dir fsync,
cleanup-on-failure. Sole format consumer is a JSON.parse reader, so the
dropped trailing newline is unobservable. +2 store-level regression tests
pin 0600 / complete JSON / no temp leftovers under burst mutations.
Judge: GLM-5.3 cold read, round-1 A, deleg_7177d506.
Verdict: urn:ump:4kwenywtf3nx2mokobvuzrhklk2xigmpyea6nvqdyhv7icnflpkq
Full suite: 121 suites / 2770 tests green.

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

@@ -196,3 +196,42 @@ describe('DC-099 atomic-write syscall discipline (mocked fs)', () => {
     expect(calls).toEqual(['openSync', 'unlinkSync']);
   });
 });
+
+// DC-100: invite-store migrated off its private _atomicWriteJSON copy onto
+// the canonical writer. Store-level pins: writes are durable-canonical
+// (0600, complete JSON, no temp leftovers) even under back-to-back mutations
+// — the access pattern that could collide tmp names in the naive copy.
+describe('DC-100 invite-store on canonical atomic-write (real fs)', () => {
+  let dir, store;
+
+  beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'dc100-invite-'));
+    store = require('../src/security/invite-store').createInviteStore({ dataDir: dir });
+  });
+  afterEach(() => { try { fs.rmSync(dir, { recursive: true, force: true }); } catch (_) {} });
+
+  test('issued invite lands as complete JSON at mode 0600 with no temp leftovers', async () => {
+    const r = await store.issue({ email: 'dc100@x.com', ttlMs: 60_000 });
+    expect(r.ok).toBe(true);
+    const file = path.join(dir, 'invites.json');
+    const st = fs.statSync(file);
+    expect(st.mode & 0o777).toBe(0o600);
+    const data = JSON.parse(fs.readFileSync(file, 'utf8'));
+    expect(Object.keys(data.invites)).toHaveLength(1);
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'invites.json');
+    expect(leftovers).toEqual([]);
+  });
+
+  test('back-to-back mutations (issue, revoke, issue) never collide on tmp names', async () => {
+    const a = await store.issue({ email: 'a@x.com', ttlMs: 60_000 });
+    const b = await store.issue({ email: 'b@x.com', ttlMs: 60_000 });
+    await store.revoke(a.id);
+    const c = await store.issue({ email: 'c@x.com', ttlMs: 60_000 });
+    expect(b.ok).toBe(true);
+    expect(c.ok).toBe(true);
+    const data = JSON.parse(fs.readFileSync(path.join(dir, 'invites.json'), 'utf8'));
+    expect(Object.keys(data.invites).sort()).toEqual([b.id, c.id].sort());
+    const leftovers = fs.readdirSync(dir).filter((f) => f !== 'invites.json');
+    expect(leftovers).toEqual([]);
+  });
+});