v5.9.0 – Per-billing-period tracking & accurate cost accounting

Highlights

Track energy cost per billing period with figures that match your actual bill — and that don't drift when the ENGIE price changes.

Added

• Billing-period history — each contract year is recorded with its consumption (kWh, m³) and a frozen cost captured at close, so closed periods always match what you were billed. New sensors: *_billing_period_history and *_cost_this_billing_period for electricity and gas.
• Close a billing period — manual or automatic. Services close_electricity_billing_period / close_gas_billing_period (with optional period_end for backdating early meter reads) and matching undo_* services. Periods also roll over automatically on the contract anniversary (a manual early close suppresses the automatic one).
• Period-end date pickers (date entities) to set the exact billing-period end before closing.
• Continuous cost accumulator — *_cost_accumulated sensors price each kWh at the rate in force when it was consumed (like the HA Energy dashboard), so they don't re-price history. Accurate from first run; not retroactive.
• Ready-made dashboard at dashboards/belgium_energy_costs_dashboard.yaml (built-in cards only) with a year-over-year comparison table. See the file header for import + the one required entity-ID edit.

Notes

• Electricity and gas are tracked independently (different anniversaries supported).
• Everything is additive and fail-safe — a failure in the new code is logged and skipped, leaving the core cost sensors unaffected.
• Existing entity IDs are unchanged; dashboards and history carry over.
• The accumulator counts forward from when you update; past periods are covered by the frozen history.