feat: optional auth + backup/restore reliability fixes

Auth is now optional: when `auth.api_keys` is empty, all endpoints are
open (no login screen, no Bearer tokens). Health endpoint reports
`auth_required` so the frontend knows which mode to use.

Backup/restore fixes:
- Auto-backup uses atomic writes (was `write_text`, risked corruption)
- Startup backup skipped if recent backup exists (<5 min cooldown),
  preventing rapid restarts from rotating out good backups
- Restore rejects all-empty backups to prevent accidental data wipes
- Store saves frozen after restore to prevent stale in-memory data
  from overwriting freshly-restored files before restart completes
- Missing stores during restore logged as warnings
- STORE_MAP completeness verified at startup against StorageConfig

server/src/wled_controller/core/backup/auto_backup.py

@@ -3,7 +3,7 @@
 import asyncio
 import json
 import os
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 from pathlib import Path
 from typing import Any, Dict, List, Optional
 
@@ -18,6 +18,11 @@ DEFAULT_SETTINGS = {
     "max_backups": 10,
 }
 
+# Skip the immediate-on-start backup if a recent backup exists within this window.
+# Prevents rapid restarts from flooding the backup directory and rotating out
+# good backups.
+_STARTUP_BACKUP_COOLDOWN = timedelta(minutes=5)
+
 
 class AutoBackupEngine:
     """Creates periodic backups of all configuration stores."""
@@ -83,11 +88,28 @@ class AutoBackupEngine:
             self._task.cancel()
             self._task = None
 
+    def _most_recent_backup_age(self) -> timedelta | None:
+        """Return the age of the newest backup file, or None if no backups exist."""
+        files = list(self._backup_dir.glob("*.json"))
+        if not files:
+            return None
+        newest = max(files, key=lambda p: p.stat().st_mtime)
+        mtime = datetime.fromtimestamp(newest.stat().st_mtime, tz=timezone.utc)
+        return datetime.now(timezone.utc) - mtime
+
     async def _backup_loop(self) -> None:
         try:
-            # Perform first backup immediately on start
-            await self._perform_backup()
-            self._prune_old_backups()
+            # Skip immediate backup if a recent one already exists.
+            # Prevents rapid restarts (crashes, restores) from flooding the
+            # backup directory and rotating out good backups.
+            age = self._most_recent_backup_age()
+            if age is None or age > _STARTUP_BACKUP_COOLDOWN:
+                await self._perform_backup()
+                self._prune_old_backups()
+            else:
+                logger.info(
+                    f"Skipping startup backup — most recent is only {age.total_seconds():.0f}s old"
+                )
 
             interval_secs = self._settings["interval_hours"] * 3600
             while True:
@@ -133,8 +155,7 @@ class AutoBackupEngine:
         filename = f"ledgrab-autobackup-{timestamp}.json"
         file_path = self._backup_dir / filename
 
-        content = json.dumps(backup, indent=2, ensure_ascii=False)
-        file_path.write_text(content, encoding="utf-8")
+        atomic_write_json(file_path, backup)
 
         self._last_backup_time = now
         logger.info(f"Auto-backup created: {filename}")
@@ -156,7 +177,6 @@ class AutoBackupEngine:
     def get_settings(self) -> dict:
         next_backup = None
         if self._settings["enabled"] and self._last_backup_time:
-            from datetime import timedelta
             next_backup = (
                 self._last_backup_time + timedelta(hours=self._settings["interval_hours"])
             ).isoformat()