People: alphabetical by last_name (PI Das pinned to end); alumni year-grouped

- Backfilled last_name field on every _people/*.md (lowercased last
  whitespace token of the name field).
- Updated tools/scrape_content.py to emit last_name on future scrapes.
- people.md: current members sorted alphabetically by last_name with
  the PI (role_order: 1) appended to the end via where_exp + concat.
- Alumni now grouped explicitly by year (Liquid sort isn't reliably
  stable, so within-year alphabetical was getting reversed). Iterate
  end_year DESC; within each year sort_natural by last_name; concat in
  order; entries missing end_year fall to the bottom alphabetically.

Author: rhiju