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>
