docs: record the taxonomy work as done

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017acfNLsJpnkvH3sCZSjMJM

docs/TODO.md

@@ -104,6 +104,21 @@ Updated 2026-09-10.
       check, so expect some wrong. The original system is kept as a cross-link,
       so a wrong pick never loses a question. Worth an editorial pass.
 - [ ] **31 questions tie between equally specific conditions** — left alone.
+- [x] **Real organ systems, and symptoms grouped under them** — done 2026-09-10.
+      The 27 "systems" were disciplines; that tree is now the Topics facet, and
+      Systems is a new flat axis of 16 organ systems. `question_tags` gained
+      `parent_id`/`sort_order` (migration `e3f4a5b6c7d8`), and 726 symptoms are
+      filed under the system they present in. 3,536 stay top-level rather than
+      being forced into an approximate bucket.
+- [x] **Taxonomy editing screen** — done 2026-09-10. `/categories` is now
+      Taxonomy: five tabs (Topics, Systems, Symptoms, Diseases, Subjects), each
+      with create / rename / reparent / delete-with-move, and "+ Questions" to
+      attach questions from a bank search. Everything added lands in the question
+      bank and quiz builder immediately.
+- [x] **Article section index backfilled** — done 2026-09-10. The index was
+      empty because `_rebuild_section_index` only runs on save, so articles
+      written before it existed had no rows. 10 articles / 28 sections indexed
+      and embedded; section-scoped question links already worked.
 - [x] **Grant dimensions** — done 2026-09-10. A grant names any combination of
       exam, discipline and category, and covers what matches all of them.
 - [x] **Exam-scoped disciplines** — done 2026-09-10. A tag may be shared