Automatic and configurable keyword-index sorting

[msordi]

Related Issue(s):

• Sorting by title doesn't work for collections. #818

Description:

Adds configurable sort-field remapping and defaults collection title to text + title.keyword so collection titles are sortable without custom mapping, while preserving explicit override support and auto-detection for mapped keyword subfields.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jonhealy1]

Hi @msordi. This pr is looking really good. Thanks for working on this.

What are your thoughts on using an item-specific env var (STAC_FASTAPI_ITEMS_SORT_FIELD_REMAPS) alongside the collections-specific one and dropping the global remap variable altogether?

Because STAC Items nest fields under properties. (e.g., properties.title) while Collections store them at the root (title), a global variable can leak Item-specific paths into Collection queries if an operator sets a remap for an overlapping field name. Isolating them into ITEMS and COLLECTIONS keeps both route domains clean.