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/server/export.go

@@ -0,0 +1,75 @@
+package server
+
+import (
+	"encoding/csv"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// exportFormats maps ?format= values to file extension and content type.
+var exportFormats = map[string]struct{ ext, contentType string }{
+	"csv": {"csv", "text/csv; charset=utf-8"},
+	"txt": {"txt", "text/plain; charset=utf-8"},
+}
+
+// handleExport downloads a list's titles in rank order, one per line, with
+// nothing else: no ranks, header row or metadata.
+func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
+	format := r.URL.Query().Get("format")
+	if format == "" {
+		format = "txt"
+	}
+	f, ok := exportFormats[format]
+	if !ok {
+		http.Error(w, `format must be "csv" or "txt"`, http.StatusBadRequest)
+		return
+	}
+	l, ok := s.visibleList(w, r)
+	if !ok {
+		return
+	}
+	items, err := s.store.Items(r.Context(), l.ID, 0, 0)
+	if err != nil {
+		s.serverError(w, r, err)
+		return
+	}
+	titles := make([]string, len(items))
+	for i, it := range items {
+		titles[i] = it.Title
+	}
+	w.Header().Set("Content-Type", f.contentType)
+	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s.%s"`, l.Slug, f.ext))
+	w.Header().Set("Cache-Control", "no-store")
+	if err := writeTitles(w, format, titles); err != nil {
+		s.log.Warn("export failed", "list", l.Slug, "err", err)
+	}
+}
+
+// flatten keeps one title per line even if a title contains a newline.
+var flatten = strings.NewReplacer("\r\n", " ", "\n", " ", "\r", " ")
+
+func writeTitles(w io.Writer, format string, titles []string) error {
+	if format == "csv" {
+		// The BOM makes Excel read accented titles (Les Misérables) as UTF-8.
+		if _, err := io.WriteString(w, "\xef\xbb\xbf"); err != nil {
+			return err
+		}
+		cw := csv.NewWriter(w)
+		cw.UseCRLF = true
+		for _, t := range titles {
+			if err := cw.Write([]string{flatten.Replace(t)}); err != nil {
+				return err
+			}
+		}
+		cw.Flush()
+		return cw.Error()
+	}
+	for _, t := range titles {
+		if _, err := io.WriteString(w, flatten.Replace(t)+"\n"); err != nil {
+			return err
+		}
+	}
+	return nil
+}