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/store/store.go

@@ -86,6 +86,8 @@ CREATE TABLE items (
 );
 CREATE INDEX items_list_rank ON items(list_id, rank);
 `,
+	// Per-list artwork shape; '' uses the category's default.
+	`ALTER TABLE lists ADD COLUMN art TEXT NOT NULL DEFAULT '';`,
 }
 
 func (s *Store) migrate(ctx context.Context) error {