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.service.spec.ts

@@ -66,6 +66,20 @@ describe('MailService.postInternal', () => {
     expect(aliceThreads.map((t) => t.threadId)).toContain(res.threadId);
   });
 
+  it('stores an in-app attachment as a media part alongside the body', async () => {
+    const res = await mail().postInternal(alice, {
+      source: inline, recipientUserId: 'bob', idempotencyKey: 'int:att',
+      attachments: [{ filename: 'roof.png', contentRef: 'scope/roof.png', mimeType: 'image/png', sizeBytes: 2048 }],
+    });
+    const interaction = await prisma.iiosInteraction.findUniqueOrThrow({ where: { id: res.interactionId }, include: { parts: true } });
+    const file = interaction.parts.find((p) => p.contentRef);
+    expect(file).toBeDefined();
+    expect(file!.kind).toBe('MEDIA_REF'); // image/* → MEDIA_REF
+    expect(file!.contentRef).toBe('scope/roof.png');
+    expect(file!.mimeType).toBe('image/png');
+    expect(file!.sizeBytes != null ? Number(file!.sizeBytes) : null).toBe(2048);
+  });
+
   it('is idempotent per key — a replay reuses the same thread, no duplicate interaction', async () => {
     const a = await mail().postInternal(alice, { source: inline, recipientUserId: 'bob', idempotencyKey: 'int:dup' });
     const b = await mail().postInternal(alice, { source: inline, recipientUserId: 'bob', idempotencyKey: 'int:dup' });