Logo in system mail; "don't ask for the 2FA code on this device"

System mail: the password-reset email (and the admin panel's test email) now
opens with the Ultimo logo, centered. It is a PNG rendered from
icons/ultimo-logo.svg and embedded in the message (multipart/related, cid:),
so it shows where remote images are blocked and in clients that do not
render SVG. The PNG is listed in package-data so the Docker image carries it.

Trusted devices: a 2FA sign-in offers "Don't ask for the code on this device
in future". Ticked with a correct code, the browser gets its own HttpOnly
trusted-device cookie (token in Redis, 90 days, renewed on use); later
sign-ins to the same account from that browser need only the password. Other
accounts and browsers are unaffected. The trust is revoked on a password
reset, when 2FA is turned off, and when an admin blocks or deletes the user;
signing out keeps it.

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

backend/app/identity/mailer.py

@@ -15,6 +15,8 @@ import logging
 from dataclasses import dataclass
 from email.message import EmailMessage
 from email.utils import formataddr, make_msgid
+from functools import cache
+from pathlib import Path
 
 import aiosmtplib
 
@@ -26,6 +28,22 @@ logger = logging.getLogger(__name__)
 SENDER_NAME = "Ultimo Email"
 _TIMEOUT = 20.0
 
+# The logo travels inside the message (multipart/related, referenced by cid:),
+# not as a link: it shows even where remote images are blocked, and it is a PNG
+# because mail clients (Gmail first) do not display SVG. Rendered at twice the
+# size it is shown at, for sharp high-DPI screens.
+_LOGO_FILE = Path(__file__).parent / "assets" / "ultimo-logo.png"
+_LOGO_CID = "ultimo-logo@ultimo.email"
+LOGO_HTML = (
+    f'<p style="text-align:center;margin:0 0 24px"><img src="cid:{_LOGO_CID}" '
+    'width="180" height="47" alt="Ultimo Email" style="display:inline-block;border:0"></p>'
+)
+
+
+@cache
+def _logo() -> bytes:
+    return _LOGO_FILE.read_bytes()
+
 
 class MailerError(Exception):
     """System mail is not configured, or the SMTP server refused the message."""
@@ -67,6 +85,10 @@ async def send_mail(cfg: MailerConfig, to: str, subject: str, text: str, html: s
     msg["Auto-Submitted"] = "auto-generated"  # RFC 3834: do not auto-reply to this
     msg.set_content(text)
     msg.add_alternative(html, subtype="html")
+    if f"cid:{_LOGO_CID}" in html:
+        html_part = msg.get_body(("html",))
+        if isinstance(html_part, EmailMessage):
+            html_part.add_related(_logo(), maintype="image", subtype="png", cid=f"<{_LOGO_CID}>")
     try:
         await aiosmtplib.send(
             msg,
@@ -99,7 +121,7 @@ def reset_mail(link: str, handle: str) -> tuple[str, str, str]:
     )
     body = "font-family:Arial,Helvetica,sans-serif;font-size:15px;color:#1f2937;line-height:1.5"
     html = (
-        f'<!doctype html>\n<html><body style="{body}">\n'
+        f'<!doctype html>\n<html><body style="{body}">\n{LOGO_HTML}\n'
         f"<p>Someone asked to change the password of the Ultimo Email account <b>{h}</b>.</p>\n"
         "<p>To choose a new password, open this link within 30 minutes:</p>\n"
         f'<p><a href="{u}" style="{button}">Change password</a></p>\n'