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.dto.ts

@@ -7,6 +7,7 @@ export class AttachmentDto {
   @IsOptional() @IsString() filename?: string;
   @IsString() @IsNotEmpty() contentRef!: string;
   @IsOptional() @IsString() mimeType?: string;
+  @IsOptional() @IsInt() sizeBytes?: number;
 }
 
 /** POST /v1/mail/internal — app-to-app mail (no SMTP). Provide EITHER `key` OR `inline`. */
@@ -19,6 +20,8 @@ export class MailInternalDto {
   @IsOptional() @IsObject() vars?: Record<string, unknown>;
   @IsOptional() @IsString() locale?: string;
   @IsString() @IsNotEmpty() idempotencyKey!: string;
+  /** In-app attachment refs — stored as message parts so the recipient's inbox can render them. */
+  @IsOptional() @IsArray() @ValidateNested({ each: true }) @Type(() => AttachmentDto) attachments?: AttachmentDto[];
 }
 
 /** POST /v1/mail/send — external email via SMTP + optional in-app mirror for a registered recipient. */