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-contracts/src/ingest.ts

@@ -27,6 +27,7 @@ export interface IngestInteractionRequest {
     bodyText?: string;
     contentRef?: string;
     mimeType?: string;
+    sizeBytes?: number;
   }>;
   occurredAt: string;
   providerEventId?: string;