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

.github/workflows/test-and-deploy-ipv4only.yaml

@@ -75,8 +75,7 @@ jobs:
           cmdeploy init staging-ipv4.testrun.org
           sed -i 's#disable_ipv6 = False#disable_ipv6 = True#' chatmail.ini
           sed -i 's/#\s*mtail_address/mtail_address/' chatmail.ini
-
-      - run: cmdeploy run --verbose --skip-dns-check
+          cmdeploy run --verbose --skip-dns-check
 
       - name: set DNS entries
         run: |