feat(relay): per-channel TX throttle and live duty-cycle budget

[iceice400]

Summary

• Add per-channel rolling 1 h relay ToA budget (ChannelBudget) with operator throttle sliders; relay TX only, not native messaging.
• RelayManager.evaluate() rejects with channel_throttled when budget exhausted; records airtime after successful relay TX.
• GET /api/stats/duty_cycle + Stats tab live meter; Configuration → Advanced throttle UI; PUT /api/config/relay hot-reload.

Why

Operators need a duty-cycle overlay and per-channel relay throttle to avoid starving busy channels while staying within regional hints (EU868 1% ceiling).

Soft dependencies

• Reuses src/analytics/toa_estimate.py (PR 2 ToA math).
• Throttle UI lives in Configuration → Advanced (standalone card here; merges into PR 4 relay filters card when that lands).

Risks (request maintainer review)

• Touches RelayManager.evaluate() — misconfigured throttle can starve relay traffic on busy channels.
• ToA estimates are not spectrum-analyser measurements.
• Throttle is relay-only; transmit.enabled path unchanged.

Testing

• python -m unittest tests.test_relay_duty_cycle -v (8 tests)
• Channel 0 at 50% throttle blocks relay under synthetic load; omitted throttle = 100% behaviour.

Impact

• Default channel_throttle_percent: {} = 100% all channels (no behaviour change on upgrade).