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/pop3/server.py

@@ -16,6 +16,7 @@ from decnet_logging import SEVERITY_WARNING, syslog_line, write_syslog_file, for
 NODE_NAME    = os.environ.get("NODE_NAME", "mailserver")
 SERVICE_NAME  = "pop3"
 LOG_TARGET   = os.environ.get("LOG_TARGET", "")
+PORT         = int(os.environ.get("PORT", "110"))
 POP3_BANNER  = os.environ.get("POP3_BANNER", f"+OK {NODE_NAME} Dovecot POP3 ready.")
 _RAW_USERS   = os.environ.get("IMAP_USERS", "admin:admin123,root:toor,mail:mail,user:user")
 
@@ -405,7 +406,7 @@ class POP3Protocol(asyncio.Protocol):
 async def main():
     _log("startup", msg=f"POP3 server starting as {NODE_NAME}")
     loop = asyncio.get_running_loop()
-    server = await loop.create_server(POP3Protocol, "0.0.0.0", 110)  # nosec B104
+    server = await loop.create_server(POP3Protocol, "0.0.0.0", PORT)  # nosec B104
     async with server:
         await server.serve_forever()