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/getting_started.rst

@@ -47,6 +47,14 @@ steps. Please substitute it with your own domain.
        www.chat.example.org. 3600 IN CNAME chat.example.org.
        mta-sts.chat.example.org. 3600 IN CNAME chat.example.org.
 
+   .. note::
+
+      For experimental deployments using self-signed certificates,
+      use a domain name starting with ``_``
+      (e.g. ``_chat.example.org``).
+      The ``mta-sts`` CNAME and ``_mta-sts`` TXT records
+      are not needed for such domains.
+
 2. On your local PC, clone the repository and bootstrap the Python
    virtualenv.
 
@@ -63,6 +71,16 @@ steps. Please substitute it with your own domain.
 
        scripts/cmdeploy init chat.example.org  # <-- use your domain
 
+   To use self-signed TLS certificates
+   instead of Let's Encrypt,
+   use a domain name starting with ``_``
+   (e.g. ``scripts/cmdeploy init _chat.example.org``).
+   Domains starting with ``_`` cannot obtain WebPKI certificates,
+   so self-signed mode is derived automatically.
+   This is useful for private or test deployments.
+   See the :doc:`overview`
+   for details on certificate provisioning.
+
 4. Verify that SSH root login to the deployment server server works:
 
    ::
@@ -169,6 +187,17 @@ creating addresses, login with ssh to the deployment machine and run:
 Chatmail address creation will be denied while this file is present.
 
 
+Running a relay with self-signed certificates
+----------------------------------------------
+
+Use a domain name starting with ``_`` (e.g. ``_chat.example.org``)
+to run a relay with self-signed certificates.
+Domains starting with ``_`` cannot obtain WebPKI certificates
+so the relay automatically uses self-signed certificates
+and all other relays will accept connections from it
+without requiring certificate verification.
+This is useful for experimental setups and testing.
+
 Migrating to a new build machine
 ----------------------------------