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/api/routes/backup.py

@@ -27,6 +27,7 @@ from wled_controller.api.schemas.system import (
 )
 from wled_controller.core.backup.auto_backup import AutoBackupEngine
 from wled_controller.config import get_config
+from wled_controller.storage.base_store import freeze_saves
 from wled_controller.utils import atomic_write_json, get_logger
 
 logger = get_logger(__name__)
@@ -175,6 +176,7 @@ async def import_store(
         return len(incoming)
 
     count = await asyncio.to_thread(_write)
+    freeze_saves()
     logger.info(f"Imported store '{store_key}' ({count} entries, merge={merge}). Scheduling restart...")
     _schedule_restart()
     return {
@@ -269,6 +271,27 @@ async def restore_config(
         if not isinstance(stores[key], dict):
             raise HTTPException(status_code=400, detail=f"Store '{key}' in backup is not a valid JSON object")
 
+    # Guard: reject backups where every store is empty (version key only, no entities).
+    # This prevents accidental data wipes from restoring a backup taken when the
+    # server had no data loaded.
+    total_entities = 0
+    for key in present_keys:
+        store_data = stores[key]
+        for field_key, field_val in store_data.items():
+            if field_key != "version" and isinstance(field_val, dict):
+                total_entities += len(field_val)
+    if total_entities == 0:
+        raise HTTPException(
+            status_code=400,
+            detail="Backup contains no entity data (all stores are empty). Aborting to prevent data loss.",
+        )
+
+    # Log missing stores as warnings
+    missing = known_keys - present_keys
+    if missing:
+        for store_key in sorted(missing):
+            logger.warning(f"Restore: backup is missing store '{store_key}' — existing data will be kept")
+
     # Write store files atomically (in thread to avoid blocking event loop)
     config = get_config()
 
@@ -284,10 +307,13 @@ async def restore_config(
 
     written = await asyncio.to_thread(_write_stores)
 
+    # Freeze all store saves so the old process can't overwrite restored files
+    # with stale in-memory data before the restart completes.
+    freeze_saves()
+
     logger.info(f"Restore complete: {written}/{len(STORE_MAP)} stores written. Scheduling restart...")
     _schedule_restart()
 
-    missing = known_keys - present_keys
     return RestoreResponse(
         status="restored",
         stores_written=written,