Submit meter reading to supplier

Submit a meter reading to the supplier (Octopus Energy, E.ON Next, British Gas Lite). The reading is saved to Firestore with status PENDING first, then sent to the supplier; the document is updated to SUCCESS or FAILED. Supports single-register (gas or single-rate electricity), day/night (2 registers), or multi-register (e.g. green/amber/red). Get registerIds from the supplier account’s meter details (e.g. from account sites). On failure, the response includes readingId so you can retry within the typical 10-day submission window.