refactor(dates): condense the two date-contract migrations into one

Neither migration had shipped, so the #115 snapshot columns join the
#114 new-contract columns in 2026-09-24-000000_add_true_utc_dates.
Pure ADD COLUMN either way — population is runtime (drains), so nothing
needs re-migrating. Column order is unchanged, so schema.rs stands.

migrations/2026-09-24-000000_add_true_utc_dates/up.sql

@@ -1,4 +1,4 @@
--- True-instant date contract (ADR-0001 / spec #113, ticket #114).
+-- True-instant date contract (ADR-0001 / spec #113, tickets #114 + #115).
 --
 -- image_exif gains the new-contract columns alongside the untouched legacy
 -- `date_taken`:
@@ -10,10 +10,23 @@
 --   date_taken_tz_source  — why the offset is what it is:
 --                           offset_tag | gps | assumed_server | utc_direct
 --                           | manual.
--- user_ai_prefs gains the DST-proof display zone (IANA name); the legacy
--- tz_offset_minutes column stays frozen for rollback.
+--
+-- Manual date edits (ticket #115) write the new-contract triple alongside
+-- the legacy pair and must be revertible, so image_exif also snapshots the
+-- pre-override new-contract columns next to the existing legacy snapshot
+-- pair (original_date_taken / original_date_taken_source):
+--   original_date_utc / original_camera_offset_minutes /
+--   original_date_taken_tz_source.
+--
+-- All columns are additive and nullable; population happens at runtime
+-- (ingest + the per-tick backfill drains), so no backfill SQL is needed
+-- here. user_ai_prefs gains the DST-proof display zone (IANA name); the
+-- legacy tz_offset_minutes column stays frozen for rollback.
 
 ALTER TABLE image_exif ADD COLUMN date_utc BIGINT;
 ALTER TABLE image_exif ADD COLUMN camera_offset_minutes INTEGER;
 ALTER TABLE image_exif ADD COLUMN date_taken_tz_source TEXT;
+ALTER TABLE image_exif ADD COLUMN original_date_utc BIGINT;
+ALTER TABLE image_exif ADD COLUMN original_camera_offset_minutes INTEGER;
+ALTER TABLE image_exif ADD COLUMN original_date_taken_tz_source TEXT;
 ALTER TABLE user_ai_prefs ADD COLUMN display_tz TEXT;