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/pages/list.html

@@ -7,7 +7,7 @@
     <a class="back-link" href="/">{{icon "arrow-left"}}All lists</a>
     <div class="list-hero-row">
       <div>
-        <p class="eyebrow">{{icon .Category.Icon}}{{.Category.Label}}</p>
+        {{if ne .Category.Key "custom"}}<p class="eyebrow">{{icon .Category.Icon}}{{.Category.Label}}</p>{{end}}
         <h1 class="list-title">{{.List.Title}}</h1>
         {{with .List.Description}}<p class="list-desc">{{.}}</p>{{end}}
         <p class="list-meta">
@@ -16,7 +16,7 @@
         </p>
       </div>
       <div class="list-actions">
-        {{if $.LoggedIn}}<a class="btn btn--ghost" href="/admin/lists/{{.List.ID}}">{{icon "pencil"}}Edit</a>{{end}}
+        {{if $.LoggedIn}}<a class="btn btn--ghost" href="/admin/lists/{{.List.ID}}">{{icon "pencil"}}Edit</a>{{if .Items}}{{template "export" .List.Slug}}{{end}}{{end}}
         {{if .Items}}
         <div class="segmented" role="group" aria-label="Layout" data-layout-toggle data-default="{{.List.Layout}}" data-key="{{.List.Slug}}">
           <button type="button" data-layout="grid" title="Grid">{{icon "grid"}}<span class="sr-only">Grid</span></button>