Add on-demand media URL refresh for expired MLS Grid tokens

MLS Grid media URLs expire after ~24 hours. Instead of running
scheduled syncs, this adds on-demand refresh when images are requested:

- Add is_url_expired() to parse expires timestamp from media URLs
- Add refresh_media_urls() to fetch fresh URLs from API for one listing
- Add get_property_media() API method using ListingId filter
- Image endpoint checks URL expiration before fetching
- If expired, refreshes URLs from API then proceeds with fetch

This is more efficient than scheduled full syncs because:
- Only refreshes URLs for listings actually being viewed
- Zero overhead for unviewed listings
- Scales naturally with traffic

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

wp-content/plugins/mls-by-hansonxyz/includes/class-mls-api-client.php

@@ -378,6 +378,42 @@ class MLS_API_Client {
         return $this->request('Property', $params);
     }
 
+    /**
+     * Get a single property by listing ID with media
+     *
+     * Used to refresh media URLs for a specific listing without
+     * fetching the entire dataset.
+     *
+     * Note: MLS Grid only allows filtering by ListingId (not ListingKey)
+     * for the Property resource. The caller must provide the listing_id.
+     *
+     * @param string $listing_id The MLS listing ID (not listing_key)
+     * @return array|WP_Error Property data with Media or error
+     */
+    public function get_property_media($listing_id) {
+        $params = array();
+
+        $system = $this->options->get_originating_system();
+
+        // Filter by ListingId (MLS Grid only allows certain fields for filtering)
+        $params['$filter'] = "OriginatingSystemName eq '{$system}' and ListingId eq '{$listing_id}'";
+        $params['$expand'] = 'Media';
+        $params['$top'] = 1;
+
+        $response = $this->request('Property', $params);
+
+        if (is_wp_error($response)) {
+            return $response;
+        }
+
+        // Return first result or null
+        if (isset($response['value']) && !empty($response['value'])) {
+            return $response['value'][0];
+        }
+
+        return null;
+    }
+
     /**
      * Get next page of results
      *