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/config/default_config.yaml

@@ -8,11 +8,11 @@ server:
     - "http://localhost:8080"
 
 auth:
-  # API keys are REQUIRED - authentication is always enforced
-  # Format: label: "api-key"
+  # API keys — when empty, authentication is disabled (open access).
+  # To enable auth, add one or more label: "api-key" entries.
+  # Generate secure keys: openssl rand -hex 32
   api_keys:
-    # Generate secure keys: openssl rand -hex 32
-    dev: "development-key-change-in-production"  # Development key - CHANGE THIS!
+    dev: "development-key-change-in-production"
 
 storage:
   devices_file: "data/devices.json"