refactor(persistence): canonical atomic file writer + notifications.json crash-safety (DC-099) [glm-grade=B]

- src/utils/atomic-write.js: single shared tmp+fsync+rename writer
  (exclusive-create 0600, unique tmp names, cleanup-on-failure,
  best-effort parent-dir fsync after rename for swap durability)
- notification-manager: both write paths (load-time canonicalization
  write-back + saveConfig) converted from plain writeFileSync — a
  crash mid-write can no longer truncate notifications.json
- DC-097/098 test seams migrated to the atomic path; new suite pins
  syscall discipline (order, wx flags, tmp naming, error cleanup,
  dir-fsync swallow)
- 121 suites / 2768 tests green
- Judge: GLM-5.3 cold read grade B/ship (deleg_23f7abad); polish items
  folded: dir-fsync added, header copy-count corrected. Remaining:
  migrate invite/user/share-store private _atomicWriteJSON copies as
  they are touched (queued).

URN: pending (recorded post-commit)

dashcaddy-api/src/managers/notification-manager.js

@@ -6,6 +6,7 @@ const EventEmitter = require('events');
 const fs = require('fs');
 const path = require('path');
 const nodemailer = require('nodemailer');
+const { atomicWriteJSON } = require('../utils/atomic-write');
 
 // Canonical event names are kebab-case ('container-down'). Emitters and the
 // settings UI historically send camelCase ('containerDown', 'deploymentSuccess')
@@ -125,7 +126,9 @@ class NotificationManager extends EventEmitter {
     try {
       const canonical = JSON.stringify(this.config, null, 2);
       if (canonical !== rawFileContents) {
-        fs.writeFileSync(this.NOTIFICATIONS_FILE, canonical);
+        // DC-099: tmp+fsync+rename — a crash mid-write can no longer leave a
+        // truncated/empty notifications.json (plain writeFileSync could).
+        atomicWriteJSON(this.NOTIFICATIONS_FILE, this.config);
         this.log.info?.('notification', 'Notification config canonicalized on disk (legacy keys normalized)', {});
       }
     } catch (writeError) {
@@ -198,7 +201,9 @@ class NotificationManager extends EventEmitter {
       if (!fs.existsSync(dir)) {
         fs.mkdirSync(dir, { recursive: true });
       }
-      fs.writeFileSync(this.NOTIFICATIONS_FILE, JSON.stringify(this.config, null, 2));
+      // DC-099: atomic tmp+fsync+rename — the UI save path gets the same
+      // crash-safety as the load-path write-back (no torn notifications.json).
+      atomicWriteJSON(this.NOTIFICATIONS_FILE, this.config);
       return true;
     } catch (error) {
       this.log.error('notification', error, null, { note: 'Failed to save config' });