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/schemas/system.py

@@ -13,6 +13,7 @@ class HealthResponse(BaseModel):
     timestamp: datetime = Field(description="Current server time")
     version: str = Field(description="Application version")
     demo_mode: bool = Field(default=False, description="Whether demo mode is active")
+    auth_required: bool = Field(default=True, description="Whether API key authentication is required")
 
 
 class VersionResponse(BaseModel):