Account for Chile DST in "night" searches

[phirstgemini]

We currently have a fixed CHILE_OFFSET defined in gemini_metadata_utils/datetimes.py

It would be better to have postgres do this properly using existing timezone definitions:

phirst=> select timestamp with time zone '2026-09-15 14:00:00 Chile/Continental';
      timestamptz       
------------------------
 2026-09-15 17:00:00+00
(1 row)

phirst=> select timestamp with time zone '2026-08-15 14:00:00 Chile/Continental';
      timestamptz       
------------------------
 2026-08-15 18:00:00+00
(1 row)