feat: batch media metadata updates and delete with usage check

lit/media-update validates the whole batch first (edit_post, lengths, optional
modified lock) and restores already written items when a write fails midway.
lit/media-delete requires confirm, refuses used media without force and
reports refused deletes as write_failed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Media/MediaPresenter.php

@@ -29,6 +29,8 @@ class MediaPresenter
             'height' => isset($meta['height']) ? (int) $meta['height'] : null,
             'filesize' => isset($meta['filesize']) ? (int) $meta['filesize'] : null,
             'date' => (string) $attachment->post_date,
+            // Optimistic lock for lit/media-update (post_modified_gmt, changes with every row write).
+            'modified' => (string) $attachment->post_modified_gmt,
         ];
     }
 
@@ -77,6 +79,7 @@ class MediaPresenter
                 'height' => ['type' => ['integer', 'null']],
                 'filesize' => ['type' => ['integer', 'null']],
                 'date' => ['type' => 'string'],
+                'modified' => ['type' => 'string'],
             ],
             'required' => ['id', 'title', 'mime_type', 'url', 'alt'],
         ];