Add optional MQTT automation relay for non-admin users (#117)

[eyalgal]

v2.9.1 adds optional MQTT publishing for non-admin users on Home Assistant 2026.10+, addressing #117.

Improvements

• Non-admin MQTT relay: mqtt.publish_automation routes timer storage, state updates, and timer events through an administrator-configured automation. Admin users keep direct publishing.
• Visual editor support: select the relay under Timer storage.
• Clear permission errors: denied requests pause further writes instead of repeatedly calling the unauthorized action. Missing relay configuration shows a setup warning.
• Compatibility preserved: both MQTT storage formats remain supported, and non-admin users on known HA versions before 2026.10 retain direct publishing when no relay is configured.
• Existing data preserved: adding the relay setting does not change automatic storage namespaces or pinned IDs. There is no automatic fallback to local storage.

Non-admin publishing on HA 2026.10+ requires the optional backend relay. See the MQTT guide for configuration details.