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-settings.js

@@ -306,8 +306,8 @@ function integrationsSection() {
       ready: () => true,
     }),
     integration({
-      title: "Theater · Wikipedia", iconName: "masks", category: "theater",
-      desc: "No key needed. Finds musicals, plays and operas on Wikipedia. You choose the production (Broadway, a tour…) and can swap in its own poster.",
+      title: "Theater & Custom · Wikipedia", iconName: "masks", category: "theater",
+      desc: "No key needed. Theater lists search musicals, plays and operas; you choose the production (Broadway, a tour…) and can swap in its own poster. Custom lists can search all of Wikipedia for a name and picture, or skip search and add entries by hand.",
       ready: () => true,
     }),
   );