feat: sitemap settings singleton with content types and additional addresses

- one settings tile opening the golem15.sitemap.settings singleton
- Files tab (site address, file size, S1 files partial), Additional
  addresses repeater, Content types tab built from signpost at boot
- golem15_sitemap_types; a type switched off leaves the files (reason type)
- a save validates (errors.*) and dispatches a rebuild after commit

updates/202610280003_create_golem15_sitemap_types.go

@@ -0,0 +1,31 @@
+package updates
+
+import (
+	"github.com/go-gormigrate/gormigrate/v2"
+	"gorm.io/gorm"
+)
+
+// createTypes creates golem15_sitemap_types: one row per signpost content
+// type the settings' Content types tab saved (include, file slug, HTML
+// sitemap heading per locale). A type without a row uses the defaults.
+var createTypes = []*gormigrate.Migration{
+	{
+		ID: "202610280003_create_golem15_sitemap_types",
+		Migrate: func(tx *gorm.DB) error {
+			return tx.Exec(`CREATE TABLE golem15_sitemap_types (
+	type_key TEXT PRIMARY KEY,
+	include BOOLEAN NOT NULL DEFAULT TRUE,
+	slug TEXT NOT NULL DEFAULT '',
+	titles JSONB NULL,
+	updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+)`).Error
+		},
+		Rollback: func(tx *gorm.DB) error {
+			return tx.Exec(`DROP TABLE IF EXISTS golem15_sitemap_types`).Error
+		},
+	},
+}
+
+func init() {
+	Register(createTypes...)
+}