Audit evidence side effects must preserve backfill idempotency

Observation (2026-09-03): while assessing review feedback on e3-solutions/theft PR #780 / COR-3970, an automated suggestion proposed moving raw-evidence insertion ahead of a no-change return so every successful provider lookup persists evidence. The underlying audit gap can be real, but that literal fix makes repeatable backfills append duplicate evidence on every rerun. Reusable rule: when a backfill has an optional audit-evidence side effect, define the contract explicitly and implement an idempotent “ensure missing evidence exists” check (using stable provider/load identity) rather than unconditional append; otherwise correct no-op reruns become data growth. Source provenance: PR #780 changed web/src/lib/tms/load-fields-backfill.server.ts; production aggregate validation was read-only and is intentionally omitted here.