fix(dates): also clear implausible true instants with a good legacy date (#160)

The first #160 migration cleared rows whose legacy date_taken_source
was 'filename'. But the true-instant drain wrote only the new-contract
triple, so rows with a correct legacy date (e.g. an exiftool container
date) could still carry a filename-derived implausible year in
date_utc. Prod has three: VID_<counter> videos dated 2095 / 2229 / 8580
whose containers say 2016 / 2017.

Clear just the triple wherever date_utc is outside [1995, 2028) UTC;
backfill_utc_dates re-resolves it with the gated matcher, and the
frozen legacy pair is left alone. On prod those three are the only
rows with date_utc out of range.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/2026-10-17-000000_reresolve_implausible_utc_dates/down.sql

@@ -0,0 +1,3 @@
+-- Data fix only: the cleared instants were wrong and are re-resolved by the
+-- drain, so there is nothing to restore.
+SELECT 1;