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/accounts/testing.py

@@ -14,6 +14,7 @@ import aiosmtplib
 
 from app.accounts.ssrf import SsrfError, assert_host_is_public
 from app.accounts.types import ServerConfig
+from app.config import smtp_helo_name
 from app.providers.base import ProviderAuthError, ProviderConnectError
 from app.providers.imap_smtp import ImapSmtpProvider, ProviderConfig
 
@@ -53,6 +54,7 @@ async def _test_smtp(config: ServerConfig, username: str, password: str) -> None
         use_tls=config.tls == "ssl",
         start_tls=config.tls == "starttls",
         timeout=_SMTP_TIMEOUT,
+        local_hostname=smtp_helo_name(),
     )
     try:
         await smtp.connect()