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/threads/threads.service.ts

@@ -9,7 +9,7 @@ export interface ThreadMessage {
   actorId: string | null;
   kind: string;
   occurredAt: Date;
-  parts: Array<{ kind: string; bodyText: string | null; contentRef: string | null }>;
+  parts: Array<{ kind: string; bodyText: string | null; contentRef: string | null; mimeType: string | null; sizeBytes: number | null }>;
 }
 
 @Injectable()
@@ -39,7 +39,7 @@ export class ThreadsService {
         actorId: i.actorId,
         kind: i.kind,
         occurredAt: i.occurredAt,
-        parts: i.parts.map((p) => ({ kind: p.kind, bodyText: p.bodyText, contentRef: p.contentRef })),
+        parts: i.parts.map((p) => ({ kind: p.kind, bodyText: p.bodyText, contentRef: p.contentRef, mimeType: p.mimeType, sizeBytes: p.sizeBytes != null ? Number(p.sizeBytes) : null })),
       })),
     };
   }