Server+Device: push/webhook refresh to device

[fberrez]

Let the server tell a device to refresh on change, instead of poll-only, so studio edits show on the panel in seconds.

Scope

• A lightweight push channel (long-poll / SSE / a short-TTL "dirty" flag the device checks) — must survive a Pi Zero 2 W and flaky LAN.
• Server marks a device dirty when its active dashboard/config changes.
• Device refreshes promptly when signalled; falls back to the normal interval otherwise.