Make delayed AI costs converge to one canonical ledger

[farzanmrz]

Problem

Delayed gateway pricing can repair model_calls.cost_usd, but the matching usage_events.cost_usd remains whatever was available at write time—usually null or zero for BYOK calls. The two ledgers therefore permanently disagree after reconciliation.

reconcileMissingCosts() has the correct model_calls.generation_id, but usage_events has no model_call_id. Its only apparent correlation key, (ref_id, kind), is not one-to-one: multiple drafting-council calls for the same source post share that pair. A guessed join would assign costs to the wrong event.

Production reconciliation is also coupled to the end of a later ingest delivery and can be skipped when the request's 800-second budget is nearly exhausted. Even when that sweep succeeds, it repairs only model_calls.

Impact

COGS, usage metering, margin analysis, and any plan or alert built from usage_events can understate AI spend while model_calls reports a different total. We do not currently have one canonical cost ledger that reliably converges after the gateway publishes delayed pricing.

Evidence

• lib/agent/gateway-cost.ts:149-164 documents the known permanent divergence and the missing one-to-one relationship.
• lib/agent/draft-pipeline.ts:229 stamps usage_events independently with the cost known at write time.
• app/api/ingest/route.ts:89-115 runs the only production reconciliation from post-response work and may skip it when the request budget is depleted.
• supabase/migrations/20260722044255_experiments_voice_schema.sql:107-119 defines usage_events without a model-call relationship.

Acceptance

• Designate one canonical source of truth for per-model-call cost and document which ledger downstream COGS and plan calculations must use.
• Add a reliable relationship such as usage_events.model_call_id, or remove duplicate per-call cost storage so an invalid divergent state cannot be represented.
• Do not correlate rows by (ref_id, kind); multiple council calls sharing that pair must remain distinguishable.
• Make delayed pricing converge idempotently without requiring another customer delivery to arrive.
• Define a safe migration/backfill strategy for existing rows, explicitly handling historical rows that cannot be matched without guessing.
• Test delayed BYOK pricing, multiple calls sharing one source reference, retries, genuinely free/zero-cost calls, and repeated reconciliation.
• Surface or log unresolved cost rows so permanently unpriced spend is observable.

Out of scope

Provider selection, pricing changes, subscription-plan design, and unrelated usage-event kinds are not part of this issue.