M6-T07: add metering billing engine + period job + summary + recompute
- app/services/energy_cost.py: register_at (register values at a UTC boundary),
compute_period (register deltas x active-version strategy, snapshot price +
contract_version_id, immutable unless overwrite), compute_closed_periods
(bounded lookback), recompute_range (explicit overwrite), summarize
(sum net + standing fees month/30 - heffingskorting year/365).
- Missing tibber price -> skip; missing reading -> degraded (zeroed amounts,
null version), consistently on both create and recompute paths.
- main.py: 1-minute energy-cost job (existing jobs/MQTT untouched). Tests added.
tliu93
d35ac7d752