feat: media upload by URL and base64 with whitelist and size limit

MediaImporter streams URL imports via wp_safe_remote_get with limit_response_size,
checks Content-Length, intersects the MIME whitelist with get_allowed_mime_types and
validates real content; base64 is size-checked before decoding. Server-side failures
are write_failed; a failed alt text deletes the new attachment again (or partial_write).
MediaUploads is the shared permission/result helper of both upload abilities.

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

docs/guides/overview.md

@@ -19,14 +19,14 @@ des verbundenen WordPress-Benutzers.
 
 Direkt sichtbar: `lit-guide`, `lit-site-status`, `lit-content-types`, `lit-content-query`,
 `lit-content-get`, `lit-content-create`, `lit-content-update`, `lit-post-publish`, `lit-media-search`,
-`lit-media-preview`, `lit-media-download`.
+`lit-media-preview`, `lit-media-download`, `lit-media-upload-from-url`.
 
 Über `mcp-adapter-discover-abilities` / `mcp-adapter-execute-ability` (Ability-Namen `lit/…`; in Guides und
 Hinweisen immer so geschrieben):
 Status (`lit/post-unpublish`, `lit/post-schedule`, `lit/post-trash`), Kopien (`lit/post-duplicate`,
 `lit/post-replace-with-copy`), Revisionen (`lit/revisions-list`, `lit/revision-restore`), Menüs
 (`lit/menu-list`, `lit/menu-get`, `lit/menu-create`, `lit/menu-set-items`, `lit/menu-assign-location`),
-Einstellungen (`lit/wp-settings-get`, `lit/wp-settings-update`), Medien (`lit/media-get`).
+Einstellungen (`lit/wp-settings-get`, `lit/wp-settings-update`), Medien (`lit/media-get`, `lit/media-upload`).
 
 Weitere Themen: `lit-guide` mit `topic` = `content-types` oder `workflows`.