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/mail/mail.module.ts

@@ -0,0 +1,18 @@
+import { Module } from '@nestjs/common';
+import { TemplateModule } from '../templates/template.module';
+import { InteractionsModule } from '../interactions/interactions.module';
+import { MailService } from './mail.service';
+import { MailController } from './mail.controller';
+
+/**
+ * Mail orchestration: render a template (TemplateModule) → deliver it either app-to-app (an EMAIL
+ * interaction via IngestService) or externally (TemplatedSender/SMTP) with an in-app mirror.
+ * SessionVerifier + ActorResolver are global.
+ */
+@Module({
+  imports: [TemplateModule, InteractionsModule],
+  controllers: [MailController],
+  providers: [MailService],
+  exports: [MailService],
+})
+export class MailModule {}