Add titles-only list export and Custom lists

Export
- GET /l/<list>/export?format=csv|txt downloads a list's titles in rank
  order, one per line, with no header or other fields (CSV gets a UTF-8
  BOM for Excel)
- Export menu in the list editor and, for the owner, on the public list
  page; hidden lists stay owner-only

Custom lists
- New "Custom" category for anything not built in (sports teams,
  drinks…): owner-named, slug from the title, entries added by hand or
  from an unfiltered Wikipedia search
- Per-list artwork shape (square or tall), stored in a new lists.art
  column (migration 2)
- Art can be uploaded while adding an entry, not only after

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/admin.go

@@ -60,7 +60,7 @@ func (s *Server) handleAdminList(w http.ResponseWriter, r *http.Request) {
 		s.notFound(w, r)
 		return
 	}
-	cat, _ := store.CategoryByKey(l.Category)
+	cat := listCategory(l)
 	provider := ""
 	if p, err := s.providers.For(r.Context(), l.Category); err == nil {
 		provider = p.Label()