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/providers/wikipedia.go

@@ -15,6 +15,9 @@ import (
 // specific production (Broadway, a tour, …) is chosen by the owner.
 type wikipedia struct {
 	r *Registry
+	// stage limits results to musicals, plays and operas. Custom lists
+	// search all of Wikipedia.
+	stage bool
 }
 
 func (w *wikipedia) Name() string  { return "wikipedia" }
@@ -23,6 +26,9 @@ func (w *wikipedia) Label() string { return "Wikipedia" }
 const theaterTemplates = `hastemplate:"Infobox musical|Infobox play|Infobox opera"`
 
 func (w *wikipedia) Search(ctx context.Context, query string) ([]Result, error) {
+	if !w.stage {
+		return w.search(ctx, query)
+	}
 	res, err := w.search(ctx, query+" "+theaterTemplates)
 	if err == nil && len(res) == 0 {
 		res, err = w.search(ctx, query)