feat: move zjazd tags to the material's meeting

The one-way migration 20261108140000_move_zjazd_tags_to_meetings gives
every material tagged zjazd-N without a meeting meeting N (the lowest
when it has several), never overwrites a meeting_id, logs tags naming a
missing meeting or disagreeing with meeting_id at Warn, and deletes the
exact zjazd-N tags; their links go through ON DELETE CASCADE. Rollback
is a no-op: the backup is the pg_dump taken before migrate.

plugin_test.go

@@ -79,8 +79,8 @@ func TestPluginActivates(t *testing.T) {
 	if n := len(p.Models()); n != 14 {
 		t.Errorf("Models = %d, want 14", n)
 	}
-	if n := len(p.Migrations()); n != 20 {
-		t.Errorf("Migrations = %d, want 20", n)
+	if n := len(p.Migrations()); n != 21 {
+		t.Errorf("Migrations = %d, want 21", n)
 	}
 	if !slices.ContainsFunc(p.Commands(), func(c bonfire.Command) bool { return c.Name == "bm:seed" }) {
 		t.Error("bm:seed is not registered")