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_test.go

@@ -0,0 +1,28 @@
+package server
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestWriteTitles(t *testing.T) {
+	titles := []string{"Hamilton", `Les Misérables`, `Sweeney Todd: The "Demon" Barber, of Fleet Street`, "Two\nLines"}
+
+	var txt strings.Builder
+	if err := writeTitles(&txt, "txt", titles); err != nil {
+		t.Fatal(err)
+	}
+	wantTxt := "Hamilton\nLes Misérables\nSweeney Todd: The \"Demon\" Barber, of Fleet Street\nTwo Lines\n"
+	if txt.String() != wantTxt {
+		t.Errorf("txt:\n%q\nwant\n%q", txt.String(), wantTxt)
+	}
+
+	var csv strings.Builder
+	if err := writeTitles(&csv, "csv", titles); err != nil {
+		t.Fatal(err)
+	}
+	wantCSV := "\xef\xbb\xbfHamilton\r\nLes Misérables\r\n\"Sweeney Todd: The \"\"Demon\"\" Barber, of Fleet Street\"\r\nTwo Lines\r\n"
+	if csv.String() != wantCSV {
+		t.Errorf("csv:\n%q\nwant\n%q", csv.String(), wantCSV)
+	}
+}