Add RG-15 diagnostics and sky averaging config

[DeanJ87]

What changed

• Adds RG-15 UART diagnostics and health data through firmware, REST, MQTT, and web types.
• Adds sky optics and averaging configuration support, including documentation and an ADR.
• Updates config examples, user docs, API docs, and schema validation to match the new telemetry/config surface.

Why

The RG-15 work needs richer device-side diagnostics and documented configuration so sensor state, UART health, and sky-quality averaging can be tested and operated consistently.

Validation

• pio run
• npm test
• npm run build