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/providers/imap_smtp/provider.py

@@ -20,7 +20,7 @@ import aiosmtplib
 from aioimaplib import aioimaplib
 
 from app.concurrency import loop_semaphore
-from app.config import get_settings
+from app.config import get_settings, smtp_helo_name
 from app.providers.base import (
     FlagChange,
     MailboxDelta,
@@ -579,6 +579,8 @@ class ImapSmtpProvider(MailProvider):
                     start_tls=start_tls,
                     tls_context=cfg.tls_context,
                     timeout=_CONNECT_TIMEOUT,
+                    # Our own name, not the container's random host name.
+                    local_hostname=smtp_helo_name(),
                 )
         except aiosmtplib.SMTPException as exc:
             raise _smtp_error(exc) from exc