Settle a generation offer and create (or resume) the generation

Checks the offer's invoice for settlement and creates the generation once — the same mode/prompt/aspect_ratio/references as the offer are validated again here and are what actually generate. Poll this at ~2.5 s until the invoice settles (202 while unpaid), then keep calling it to follow the job: running → done (with the image) or policy_rejected (payment retained, no refund). A failed job is re-driven on the SAME payment by the next call (redriven: true) — one payment delivers at most one image. Account-bound: an expired macaroon still resumes this account's paid claim; another account's is refused. Not gated by IMAGE_GEN_ENABLED for money purposes — with generation off and the invoice PAID, the payment is recorded and the claim stays open (503 GENERATION_UNAVAILABLE, claim_open: true). Rate limited 60/min/user. Every answer is private, no-store.