Add per-room daylight mode binary sensor

[berfarah]

Summary

• Exposes NaturalLightIsOn as a per-room HA binary_sensor for each daylight-capable light entity
• Sensors attach to existing light devices via shared HA identifiers (no new devices created)
• State hydrated from REST API on startup, polled every 30s, with optimistic updates on NL toggle commands

Design

• No new types — daylight sensors are derived from LightEntity values with FollowDaylight=true
• Polling over WebSocket — the openapi-go feedback WS (:3060) would be a whole new connection type; 30s REST polling is simple and sufficient for a mode indicator
• Optimistic on toggle ON only — after SimulateButtonPress for NL toggle, we set daylight=true immediately; we don't optimistically set false on OFF since other loads in the room may still be NL-active

Test plan

• go test ./... — all existing + new tests pass
• go vet ./... — clean
• Deploy to Savant host, verify HA discovers binary_sensor attached to daylight light devices
• Toggle daylight mode from physical keypad → sensor updates within 30s
• Send ON command from HA for a daylight load → sensor updates immediately (optimistic)

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