Metered billing write surface.

Implements Accrue.Billing.report_usage/3 as a sync-through with a transactional-outbox audit table:

1. Repo.transact/2 inserts a pending %MeterEvent{} + an accrue_events ledger row, then commits.
2. Outside the transaction, calls the configured processor's report_meter_event/1 callback.
3. On {:ok, _} → flips the row to reported (and stamps reported_at). On {:error, _} → Accrue.Billing.MeterEvents performs a guarded pending → failed transition and emits [:accrue, :ops, :meter_reporting_failed] at most once for that transition (source: :sync).

Crashes between step 1 and step 2 leave a durable pending row that Accrue.Jobs.MeterEventsReconciler retries on its next cron tick.

Idempotency: the caller's operation_id (via Accrue.Actor) + event name + value + timestamp derive a deterministic identifier. The unique index on accrue_meter_events.identifier dedupes at the audit layer; Stripe's body-level identifier + HTTP-level idempotency_key (forwarded in Accrue.Processor.Stripe) dedupe at the wire.