date_taken_source: backfill 'exif' on legacy rows

Pre-resolver rows already had a populated `date_taken` from the old
kamadak-exif-only ingest path. The column-add migration left their
`date_taken_source` as NULL, and the drain's eligibility predicate
(`date_taken IS NULL OR date_taken_source = 'fs_time'`) skips them —
so they remain unlabelled forever and never benefit from the
resolver's exiftool fallback even if they're videos that should
upgrade.

Label them all `'exif'` in a one-shot UPDATE. Safe because every
write path that populated `date_taken` before the resolver landed was
a kamadak-exif read. Idempotent (the WHERE matches nothing on a
second run). Down.sql is a no-op — the labels stay correct under any
schema state, and the column-add migration is the right place to
revert if needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/2026-05-06-000100_backfill_date_taken_source_legacy/down.sql

@@ -0,0 +1,9 @@
+-- Reverting this migration is a no-op: the labels we wrote in `up.sql`
+-- are correct under any state of the schema (every dated row was indeed
+-- exif-sourced before the resolver landed), and there's no signal that
+-- distinguishes "labelled by this migration" from "labelled by the
+-- ingest path post-resolver". Clearing them would break the drain's
+-- eligibility filter again.
+--
+-- The companion migration `2026-05-06-000000_add_date_taken_source` is
+-- the one to revert if you need to remove the column entirely.