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/interactions/ingest.dto.ts

@@ -1,6 +1,7 @@
 import { Type } from 'class-transformer';
 import {
   IsArray,
+  IsInt,
   IsISO8601,
   IsObject,
   IsOptional,
@@ -40,6 +41,7 @@ class PartDto {
   @IsOptional() @IsString() bodyText?: string;
   @IsOptional() @IsString() contentRef?: string;
   @IsOptional() @IsString() mimeType?: string;
+  @IsOptional() @IsInt() sizeBytes?: number;
 }
 
 /** Validates the POST /v1/interactions/ingest body (conforms to IngestInteractionRequest). */