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/config.py

@@ -21,7 +21,7 @@ class ServerConfig(BaseSettings):
 class AuthConfig(BaseSettings):
     """Authentication configuration."""
 
-    api_keys: dict[str, str] = {}  # label: key mapping (required for security)
+    api_keys: dict[str, str] = {}  # label: key mapping (empty = auth disabled)
 
 
 class StorageConfig(BaseSettings):