feat: support self-signed chatmail relays (#855)

feat: support self-signed TLS via underscore domain convention
Domains starting with "_" (e.g. _chat.example.org) automatically use
self-signed TLS certificates instead of ACME/Let's Encrypt. The TLS
mode is derived from the domain name — no separate config option needed.

Internally, when config.tls_cert_mode is "self" (underscore domain):
- Generate self-signed certificates via openssl
- Set Postfix smtp_tls_security_level to "encrypt" (opportunistic TLS)
- Add smtp_tls_policy_map entry for underscore domains
- Skip ACME, MTA-STS and www CNAME checks in `cmdeploy dns`
- Serve /new via GET (not redirect to dcaccount:) with rate-limiting
  (nginx limit_req, 2r/s burst=5)
- Return dclogin: URLs with ic=3 (AcceptInvalidCertificates) from /new
- Render QR codes client-side via JavaScript and qrcode-svg
- Use config.tls_cert_path/tls_key_path in Postfix, Dovecot and nginx
  templates instead of hardcoded ACME paths

doc/source/overview.rst

@@ -297,8 +297,7 @@ TLS requirements
 
 Postfix is configured to require valid TLS by setting
 `smtp_tls_security_level <https://www.postfix.org/postconf.5.html#smtp_tls_security_level>`_
-to ``verify``. If emails don’t arrive at your chatmail relay server, the
-problem is likely that your relay does not have a valid TLS certificate.
+to ``verify``.
 
 You can test it by resolving ``MX`` records of your relay domain and
 then connecting to MX relays (e.g ``mx.example.org``) with
@@ -317,6 +316,14 @@ default Exim does not log sessions that are closed before sending the
 by Postfix, so you might think that connection is not established while
 actually it is a problem with your TLS certificate.
 
+If emails don’t arrive at your chatmail relay server, the
+problem is likely that your relay does not have a valid TLS certificate.
+
+Note that connections to relays with underscore-prefixed test domains
+(e.g. ``_chat.example.org``) use ``encrypt`` tls security level,
+because such domains cannot obtain valid Let's Encrypt certificates
+and run with self-signed certificates.
+
 
 .. _dovecot: https://dovecot.org
 .. _postfix: https://www.postfix.org