feat(iios): email adapter dedup fix + smoke + verification

Adds adapter-owned externalEventId extraction to the ChannelAdapter contract
(WebhookAdapter→eventId, EmailAdapter→Message-ID) and uses it in InboundService,
so email replays dedup on Message-ID (previously only payload.eventId worked).
Adds an email-dedup spec + smoke-email.mjs proving inbound → normalized, a reply
joins the same email thread, bad signature rejected, and outbound EMAIL SENT.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/src/adapters/adapters.spec.ts

@@ -113,6 +113,17 @@ describe('Email adapter — inbound + threading', () => {
     expect(await prisma.iiosThread.count()).toBe(1); // one email conversation
   });
 
+  it('redelivered email (same Message-ID) → one raw event, one interaction (dedup)', async () => {
+    const svc = inbound();
+    const { body, headers } = signedFixture(emailInboundFixture, SECRET);
+    await svc.receive('EMAIL', body, headers);
+    const second = await svc.receive('EMAIL', body, headers);
+    expect(second.deduped).toBe(true);
+    await normalizeAll();
+    expect(await prisma.iiosInboundRawEvent.count()).toBe(1);
+    expect(await prisma.iiosInteraction.count()).toBe(1);
+  });
+
   it('bad signature on EMAIL → REJECTED, nothing ingested (fail-closed)', async () => {
     const { body } = signedFixture(emailInboundFixture, SECRET);
     await expect(inbound().receive('EMAIL', body, { 'x-iios-signature': 'sha256=bad' })).rejects.toThrow();