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/config.py

@@ -74,6 +74,9 @@ class Settings(BaseSettings):
     # New passwords are checked against known data breaches (Have I Been Pwned's
     # k-anonymity range API: only 5 hex digits of a SHA-1 leave the server).
     password_breach_check: bool = Field(default=True, alias="PASSWORD_BREACH_CHECK")
+    # The name the webmail gives itself when it connects to a mail server to
+    # send (SMTP EHLO). Empty: APP_DOMAIN — not the container's random name.
+    smtp_helo_name: str = Field(default="", alias="SMTP_HELO_NAME")
     # Auto-purge Trash/Spam messages older than this many days (0 disables).
     trash_retention_days: int = Field(default=30, alias="TRASH_RETENTION_DAYS")
 
@@ -109,3 +112,9 @@ class Settings(BaseSettings):
 def get_settings() -> Settings:
     """Return the process-wide settings singleton."""
     return Settings()  # values are sourced from the environment
+
+
+def smtp_helo_name() -> str:
+    """The name to introduce ourselves with to SMTP servers (EHLO)."""
+    settings = get_settings()
+    return settings.smtp_helo_name.strip() or settings.app_domain