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/templates/partials/export.html

@@ -0,0 +1,9 @@
+{{define "export"}}
+<details class="menu">
+  <summary class="btn btn--ghost">{{icon "download"}}Export</summary>
+  <div class="menu-pop" role="menu">
+    <a role="menuitem" href="/l/{{.}}/export?format=csv" data-bind="export-csv" download>Titles as CSV <span class="muted">.csv</span></a>
+    <a role="menuitem" href="/l/{{.}}/export?format=txt" data-bind="export-txt" download>Titles as text <span class="muted">.txt</span></a>
+  </div>
+</details>
+{{end}}