The 'add' action runs due_date through parse_due_for_user (natural
language like 'tomorrow at 9am', plus user-tz anchoring for naive ISO),
but 'update' stored the raw value verbatim. A reminder edited with
natural language was saved as an unparseable literal the frontend's
new Date() can't read, so it never fired. Route update's due_date
through the same parser as add.
