✨feature: Add reset-cache action to settings page

Adds a 'Reset cache' button to the Podcast RSS Import settings page that
clears the SimplePie feed-cache rows (both the cached XML and the
modification timestamp) plus the importer's per-feed import lock for the
configured feed. Multisite site-transient variants are also cleared.

Use this when the importer is skipping items or refusing to re-fetch a
feed that has changed at the source. The next 'Import now' click will
re-pull the XML from the network.

clear_feed_cache() is exposed as a public method so it is unit-testable,
and a do_action hook (rss2cpt_cache_cleared) lets other code react when
the cache is cleared. Two smoke tests pin down the expected option and
site-transient names, and a third ensures an empty feed URL is a no-op.

tests/bootstrap.php

@@ -55,5 +55,48 @@ function get_gmt_from_date( $date_string ) {
 	return gmdate( 'Y-m-d H:i:s', $timestamp );
 }
 
+/**
+ * In-memory option store for isolated tests.
+ *
+ * @param string $name Option name.
+ * @return bool Always true; the test asserts via $GLOBALS['rss2cpt_test_deleted_options'].
+ */
+function delete_option( $name ) {
+	$GLOBALS['rss2cpt_test_deleted_options'][] = $name;
+	return true;
+}
+
+/**
+ * In-memory site-transient store for isolated tests.
+ *
+ * @param string $name Transient name.
+ * @return bool Always true; the test asserts via $GLOBALS['rss2cpt_test_deleted_site_transients'].
+ */
+function delete_site_transient( $name ) {
+	$GLOBALS['rss2cpt_test_deleted_site_transients'][] = $name;
+	return true;
+}
+
+/**
+ * Reset recorded delete_* call logs between tests.
+ *
+ * @return void
+ */
+function rss2cpt_test_reset_delete_logs() {
+	$GLOBALS['rss2cpt_test_deleted_options']         = array();
+	$GLOBALS['rss2cpt_test_deleted_site_transients'] = array();
+}
+rss2cpt_test_reset_delete_logs();
+
+/**
+ * Minimal do_action stub for isolated tests.
+ *
+ * @param string $hook Hook name.
+ * @param mixed  ...$args Hook arguments.
+ */
+function do_action( $hook, ...$args ) {
+	unset( $hook, $args );
+}
+
 require_once dirname( __DIR__ ) . '/src/Options.php';
 require_once dirname( __DIR__ ) . '/src/Import/Importer.php';