Record OpenRouter's reported cost in the usage ledger

[DevMortimer]

Problem

The usage ledger estimates spend from token counts at a fixed rate (DEFAULT_USD_PER_MTOK). OpenRouter reports the real cost on each Decisions response as usage.cost in USD (docs), for example "usage": { "input_tokens": 275, "output_tokens": 20, "cost": 0.00003 }. The TypeSafe backend returns no cost.

Wanted

• When a response carries a valid usage.cost, record it in the ledger, so maxUsdPerDay and status lines follow what was actually billed.
• Otherwise, keep the estimate.
• A missing or malformed cost is ignored, never an error. Response validation keeps passing either way.
• Existing public types keep estimatedUsd. New fields may be added.