Fills the permanent history gap on accounts whose SimpleFIN feed reaches back only a week. Imports attach to an account sync already discovered; they only insert, never reconcile, sweep, or overwrite anything sync owns. Deliberately does not reuse ingestTransactions: it treats its feed as authoritative and its stale-pending sweep would soft-delete every pending row absent from a backfill CSV. Date-only values anchor at noon UTC, not midnight — formatDay renders local, so midnight displayed a day early across the Americas and a month's first day would have read as the previous month while reporting under the correct one. The duplicate window follows, counting whole UTC calendar days rather than 86400s. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>