Invite a friend; handle field shows the domain; system mail fixes

- Invitations: each user can invite two friends by email (sign-up works
  through the link even while registration is closed); admin-editable
  template, subject and sender.
- Invitation sender defaults to the system-mail From address, which the
  SMTP account is allowed to send as (was postmaster@APP_DOMAIN, refused
  with 553 when the two differ).
- Sign-in, sign-up and forgot-password fields show the fixed
  @domain after the typed name; the registration status reports it.
- System mail gets a Date header; the invitation's plain-text part keeps
  its paragraphs and line breaks.
- SMTP EHLO uses SMTP_HELO_NAME / APP_DOMAIN, not the container name.
- DNS recommendations for the mail domains.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/__init__.py

@@ -16,6 +16,7 @@ def register_routers(app: FastAPI) -> None:
     from app.api.admin import router as admin_router
     from app.api.avatar import router as avatar_router
     from app.api.compose import router as compose_router
+    from app.api.invitations import router as invitations_router
     from app.api.mailboxes import router as mailboxes_router
     from app.api.messages import router as messages_router
     from app.api.plugins import router as plugins_router
@@ -46,6 +47,7 @@ def register_routers(app: FastAPI) -> None:
         admin_router,
         spellcheck_router,
         push_router,
+        invitations_router,
     ):
         app.include_router(router, prefix="/api")
     app.include_router(ws_router, prefix="/api")