feat(mail): inbox mirror + INTERNAL (app-to-app) delivery

MailService renders a template then deposits it as an EMAIL interaction on a
per-email thread (thread model: one thread per email), reusing IngestService.
Because ingest adds no participants and a thread is only visible to its
participants, it ensureParticipant()s BOTH sender and recipient — so the mirror
is actually visible.

- postInternal: app-to-app mail, no SMTP → recipient's in-app inbox.
- sendExternalWithMirror: SMTP send (TemplatedSender) + mirror an interaction
  ONLY for a registered recipient (pre-registration sends are email-only — no
  inbox exists yet). Idempotent across both the send and the mirror.
- Writes Interactions, NEVER InboxItems (the projector owns those — KG-15).
- POST /v1/mail/internal, POST /v1/mail/send. MailModule in AppModule.

Verified over HTTP: internal mail → recipient SEES the thread in their inbox;
external send → command + mirror; no-source/no-auth 400. 7 unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/src/app.module.ts

@@ -11,6 +11,7 @@ import { ThreadsModule } from './threads/threads.module';
 import { MessageModule } from './messaging/message.module';
 import { InboxModule } from './inbox/inbox.module';
 import { TemplateModule } from './templates/template.module';
+import { MailModule } from './mail/mail.module';
 import { MediaModule } from './media/media.module';
 import { NotificationModule } from './notifications/notification.module';
 import { SupportModule } from './support/support.module';
@@ -39,6 +40,7 @@ import { DevController } from './dev/dev.controller';
     MessageModule,
     InboxModule,
     TemplateModule,
+    MailModule,
     MediaModule,
     NotificationModule,
     SupportModule,