Prefetch the artifact-family admin changelists (#1346, Phase 3)

[jonfroehlich]

Phase 3 of #1346 (admin changelist audit). The artifact list pages — Publication, Talk, Poster, Video, Grant, Award — each render M2M/FK callable columns (author & speaker lists, projects, sponsor, recipients) with no prefetch, so they fired 1–3 queries per row. Now each is constant-query.

Changes

• get_queryset() prefetch on each admin for exactly what its changelist renders:
  • Publication → authors, projects
  • Talk → authors (speakers)
  • Poster → authors
  • Video → projects
  • Grant → authors (sponsor FK handled by list_select_related)
  • Award → recipients, projects
• Artifact.get_first_author_last_name rewritten to read list(self.authors.all()) instead of .exists() + .first() (both bypass a prefetch cache). authors is a SortedManyToManyField, so .all() preserves the editor-defined order — [0] is still the first author. (Used by Poster & Grant columns; kept behavior-identical.)
• PublicationAdmin.display_authors now reads the cached list instead of a [:5] queryset slice + .count() (both re-queried per row).

Heads-up / gotcha

There is no list_prefetch_related ModelAdmin option in Django (only list_select_related) — I initially used it, the tests caught that it's a silent no-op, and I moved the prefetch into get_queryset(). Worth knowing for future changelist work.

Tests (test_admin_perf_artifacts.py)

• Correctness of the two rewritten methods (first-author ordering, "Unknown" when authorless; display_authors five-names-plus-ellipsis and the no-authors case).
• A steady-state N+1 guard that each of the five prefetched changelists stays flat as rows grow 3 → 6 (these failed before the get_queryset fix — that's how the list_prefetch_related no-op was caught).

Full suite green locally (401 tests). Branches cleanly off current master (Phases 1 & 2 already merged).

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