refactor(persistence): migrate user-store to canonical atomic-write util (DC-101) [glm-grade=A]

Drops user-store's private _atomicWriteJSON copy (pid+Date.now() tmp
names, no fsync, no failure cleanup) in favor of src/utils/atomic-write.js
(DC-099 canonical: fsync'd same-dir exclusive-create 0600 tmp -> rename ->
parent-dir fsync, cleanup-on-failure). All 3 persisted files routed:
users.json, authorized-users.json, .bootstrapped sentinel. Sole consumers
are JSON.parse readers — dropped trailing newline unobservable (judge
verified repo-wide). +2 store-level regression tests pin 0600 / complete
JSON / no temp leftovers across all three files, incl. the bootstrap path
writing three files back-to-back in one login.
Judge: GLM-5.3 cold read, round-1 A, deleg_f632f05c.
Verdict: urn:ump:5fivvveqhcbkl6os4dhidchkvjjzbvi7rgj6znaovp6bfnvmcmsq
Full suite: 121 suites / 2772 tests green.

dashcaddy-api/src/security/user-store.js

@@ -29,8 +29,9 @@
  * This is recorded by writing a sentinel file `data/.bootstrapped` with the
  * admin email so we never bootstrap twice (e.g. after a restore from backup).
  *
- * Atomic writes: every persistence op writes to a .tmp file then renames.
- * process restart loses nothing in flight because rename is atomic on POSIX.
+ * Atomic writes: every persistence op goes through the canonical shared
+ * atomic-write util (DC-099) — fsync'd same-dir tmp+rename, so a crash or
+ * process restart loses nothing in flight and never leaves a torn file.
  *
  * Concurrency: a single in-process mutex serializes mutating ops. We don't
  * need cross-process locks because this API is single-instance by design.
@@ -42,6 +43,7 @@ const path = require('path');
 const fs = require('fs');
 const crypto = require('crypto');
 const platformPaths = require('../../platform-paths');
+const { atomicWriteJSON } = require('../utils/atomic-write');
 
 const ROLES = Object.freeze({
   ADMIN: 'admin',
@@ -76,12 +78,6 @@ function _resolveDataDir(opts) {
   return require('os').tmpdir();
 }
 
-function _atomicWriteJSON(filePath, data) {
-  const tmp = filePath + '.tmp.' + process.pid + '.' + Date.now();
-  fs.writeFileSync(tmp, JSON.stringify(data, null, 2) + '\n', { mode: 0o600 });
-  fs.renameSync(tmp, filePath);
-}
-
 function _readJSON(filePath, fallback) {
   try {
     const raw = fs.readFileSync(filePath, 'utf8');
@@ -133,8 +129,8 @@ function createUserStore(opts = {}) {
     return data;
   }
 
-  function _saveUsers(data) { _atomicWriteJSON(usersFile, data); }
-  function _saveAllowlist(data) { _atomicWriteJSON(allowlistFile, data); }
+  function _saveUsers(data) { atomicWriteJSON(usersFile, data); }
+  function _saveAllowlist(data) { atomicWriteJSON(allowlistFile, data); }
 
   function _bootstrapDone() {
     try { return fs.existsSync(bootstrapSentinel); }
@@ -142,7 +138,7 @@ function createUserStore(opts = {}) {
   }
 
   function _writeBootstrapSentinel(adminEmail) {
-    _atomicWriteJSON(bootstrapSentinel, {
+    atomicWriteJSON(bootstrapSentinel, {
       bootstrappedAt: _nowIso(),
       adminEmail: adminEmail.toLowerCase(),
     });