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

pyproject.toml

@@ -43,9 +43,11 @@ decnet = "decnet.cli:app"
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
-addopts = "-m 'not fuzz' -v -q -x -n logical"
+addopts = "-m 'not fuzz and not live' -v -q -x -n logical"
 markers = [
     "fuzz: hypothesis-based fuzz tests (slow, run with -m fuzz or -m '' for all)",
+    "live: live subprocess service tests (run with -m live)",
+    "live_docker: live Docker container tests (requires DECNET_LIVE_DOCKER=1)",
 ]
 filterwarnings = [
     "ignore::pytest.PytestUnhandledThreadExceptionWarning",