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>

web/static/js/admin-lists.js

@@ -53,7 +53,7 @@ function card(list) {
   const pct = Math.min(100, Math.round((list.itemCount / list.maxItems) * 100));
   const imgs = covers[list.id] || [];
   const art = imgs.length
-    ? h("span", { class: "list-card-covers" + (cat.aspect === "square" ? " is-square" : "") },
+    ? h("span", { class: "list-card-covers" + ((list.art || cat.aspect) === "square" ? " is-square" : "") },
       imgs.map((src) => h("span", { style: { backgroundImage: `url("${src}")` } })))
     : h("span", { class: "shelf-icon" }, icon(cat.icon));
   return h("li", { class: "list-card", "data-id": list.id },