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/mqtt/server.py

@@ -17,6 +17,7 @@ from decnet_logging import syslog_line, write_syslog_file, forward_syslog
 NODE_NAME = os.environ.get("NODE_NAME", "mqtt-broker")
 SERVICE_NAME   = "mqtt"
 LOG_TARGET = os.environ.get("LOG_TARGET", "")
+PORT = int(os.environ.get("PORT", "1883"))
 MQTT_ACCEPT_ALL = os.environ.get("MQTT_ACCEPT_ALL", "1") == "1"
 MQTT_PERSONA = os.environ.get("MQTT_PERSONA", "water_plant")
 MQTT_CUSTOM_TOPICS = os.environ.get("MQTT_CUSTOM_TOPICS", "")
@@ -264,7 +265,7 @@ class MQTTProtocol(asyncio.Protocol):
 async def main():
     _log("startup", msg=f"MQTT server starting as {NODE_NAME}")
     loop = asyncio.get_running_loop()
-    server = await loop.create_server(MQTTProtocol, "0.0.0.0", 1883)  # nosec B104
+    server = await loop.create_server(MQTTProtocol, "0.0.0.0", PORT)  # nosec B104
     async with server:
         await server.serve_forever()