The /contacts/:id edit form pre-fills qso_timestamp via
`Calendar.strftime(ts, "%Y-%m-%d %H:%M")` and submits whatever the
user typed back unchanged. Two failure modes resulted:
1. `diff_against_contact/2` compared that string against the stored
`%DateTime{}`, never matched, and treated every untouched submit
as a modification.
2. `build_contact_changes/2` then ran `DateTime.from_iso8601/1` on
the space-separated, no-seconds, no-Z form and crashed with
`MatchError` on `{:error, :invalid_format}` — direct owner/admin
submits raised instead of saving.
Adds `normalize_timestamp_field/2` to `normalize_proposed/1` that
parses both the form's `"YYYY-MM-DD HH:MM[:SS]"` and the strict
`"YYYY-MM-DDTHH:MM:SSZ"` shapes into a UTC `%DateTime{}` (with
`NaiveDateTime.from_iso8601` doing the heavy lifting after a small
seconds-padding pass). Empty strings drop the field; unparseable
input drops it too so a typo becomes "no changes" rather than a
500.
Graham McIntire
2365c19321