feat(tests): add live subprocess integration test suite for services

Spins up each service's server.py in a real subprocess via a free ephemeral
port (PORT env var), connects with real protocol clients, and asserts both
correct protocol behavior and RFC 5424 log output.

- 44 live tests across 10 services: http, ftp, smtp, redis, mqtt,
  mysql, postgres, mongodb, pop3, imap
- Shared conftest.py: _ServiceProcess (bg reader thread + queue),
  free_port, live_service fixture, assert_rfc5424 helper
- PORT env var added to all 10 targeted server.py templates
- New pytest marker `live`; excluded from default addopts run
- requirements-live-tests.txt: flask, twisted + protocol clients

templates/ftp/server.py

@@ -18,6 +18,7 @@ from decnet_logging import syslog_line, write_syslog_file, forward_syslog
 NODE_NAME = os.environ.get("NODE_NAME", "ftpserver")
 SERVICE_NAME   = "ftp"
 LOG_TARGET = os.environ.get("LOG_TARGET", "")
+PORT = int(os.environ.get("PORT", "21"))
 BANNER = os.environ.get("FTP_BANNER", "220 (vsFTPd 3.0.3)")
 
 def _log(event_type: str, severity: int = 6, **kwargs) -> None:
@@ -71,6 +72,6 @@ class ServerFTPFactory(FTPFactory):
 
 if __name__ == "__main__":
     twisted_log.startLogging(sys.stdout)
-    _log("startup", msg=f"FTP server starting as {NODE_NAME} on port 21")
-    reactor.listenTCP(21, ServerFTPFactory())
+    _log("startup", msg=f"FTP server starting as {NODE_NAME} on port {PORT}")
+    reactor.listenTCP(PORT, ServerFTPFactory())
     reactor.run()