feat(mail): in-app attachments on internal mail + surface media parts in thread reads

- MailInternalDto accepts attachments[]; deposit() stores each as a media
  part (image/video→MEDIA_REF, audio→VOICE_REF, else FILE_REF)
- ingest now persists part sizeBytes; contract + DTO carry it
- threads.getMessages returns mimeType + sizeBytes so mail readers can
  render inline images / file chips
- test: internal mail stores an image attachment as a MEDIA_REF part

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/src/mail/mail.controller.ts

@@ -22,6 +22,7 @@ export class MailController {
       vars: body.vars ?? {},
       ...(body.locale ? { locale: body.locale } : {}),
       idempotencyKey: body.idempotencyKey,
+      ...(body.attachments && body.attachments.length > 0 ? { attachments: body.attachments } : {}),
     });
   }