refactor(2meet-data-optimizer): resolve post_type/group/table mappings through the registry

- TMDO_Post_Fields::post_type_for_group() / groups_for_post_type() derive HivePress-style groups from their registry post_type scope (core keeps its own attachment / wc_product / nav_menu_item map); the three copy-pasted switches and the query router's POST_TYPE_GROUP_MAP use them
- Term/comment backfill, shadow verifier and CLI take their groups from the registry (TMDO_Term_Comment_Backfill::registered_groups()); installer drops the hp_* entries already covered by its table patterns; REST listings default post type is filterable (tmdo_rest_default_post_type, default 'post')
- Verify: NoHpMappingsTest guard + mapping/default tests; core unit 640 / integration 449, HP 146/49, WC 10/22 OK
- PR: PR-020e

includes/class-tmdo-rest-api.php

@@ -529,7 +529,7 @@ class TMDO_REST_API {
 	 * @return WP_REST_Response REST response.
 	 */
 	public function get_listings( WP_REST_Request $request ): WP_REST_Response {
-		$post_type = $request->get_param( 'post_type' ) ?? 'hp_listing';
+		$post_type = $request->get_param( 'post_type' ) ?? self::default_post_type();
 		if ( ! post_type_exists( $post_type ) || ! is_post_type_viewable( $post_type ) ) {
 			return new WP_REST_Response(
 				array(
@@ -1550,13 +1550,25 @@ class TMDO_REST_API {
 		return $filters;
 	}
 
+	/**
+	 * Post type GET /listings serves when the request names none.
+	 *
+	 * Filter `tmdo_rest_default_post_type` lets an integration (e.g. the
+	 * HivePress AddOn) make its own post type the default.
+	 *
+	 * @return string
+	 */
+	public static function default_post_type(): string {
+		return (string) apply_filters( 'tmdo_rest_default_post_type', 'post' );
+	}
+
 	/**
 	 * Argument definitions for GET /listings.
 	 */
 	private function listings_args(): array {
 		return array(
 			'post_type' => array(
-				'default'           => 'hp_listing',
+				'default'           => self::default_post_type(),
 				'sanitize_callback' => 'sanitize_key',
 			),
 			'per_page'  => array(