Move datetime formatting into minijinja filter

Investigate whether serialized time values can be formatted by the utility filters in https://docs.rs/minijinja-contrib/latest/minijinja_contrib/filters/fn.dateformat.html this will be much cleaner than the current approach of having a formatted version of the data before passing to template as context.

Start by reading the current approach that prepares formatted datetime values before template context is passed. Review the linked minijinja-contrib dateformat filter documentation and determine whether serialized time values can use it directly. Done means datetime formatting is handled by the utility filter rather than preformatted context data, with existing behavior preserved.

Written by the indexing model from the issue text.