Add RFC 5424 syslog logging to all service templates

- decnet/logging/syslog_formatter.py: RFC 5424 formatter (local0 facility,
  decnet@55555 SD element ID, full escaping per §6.3.3)
- decnet/logging/file_handler.py: rotating file handler (10 MB / 5 backups),
  path configurable via DECNET_LOG_FILE env var
- templates/decnet_logging.py: combined syslog_line / write_syslog_file /
  forward_syslog helper distributed to all 22 service template dirs
- All templates/*/server.py: replaced ad-hoc JSON _forward/_log with RFC 5424
  syslog_line + write_syslog_file + forward_syslog
- All templates/*/Dockerfile: COPY decnet_logging.py /opt/
- DecnetConfig: added log_file field; CLI: --log-file flag;
  composer injects DECNET_LOG_FILE env var into service containers
- tests/test_syslog_formatter.py + tests/test_file_handler.py: 25 new tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/sip/server.py

@@ -11,8 +11,10 @@ import os
 import re
 import socket
 from datetime import datetime, timezone
+from decnet_logging import syslog_line, write_syslog_file, forward_syslog
 
 NODE_NAME = os.environ.get("NODE_NAME", "pbx")
+SERVICE_NAME   = "sip"
 LOG_TARGET = os.environ.get("LOG_TARGET", "")
 
 _401 = (
@@ -27,27 +29,13 @@ _401 = (
 )
 
 
-def _forward(event: dict) -> None:
-    if not LOG_TARGET:
-        return
-    try:
-        host, port = LOG_TARGET.rsplit(":", 1)
-        with socket.create_connection((host, int(port)), timeout=3) as s:
-            s.sendall((json.dumps(event) + "\n").encode())
-    except Exception:
-        pass
 
 
-def _log(event_type: str, **kwargs) -> None:
-    event = {
-        "ts": datetime.now(timezone.utc).isoformat(),
-        "service": "sip",
-        "host": NODE_NAME,
-        "event": event_type,
-        **kwargs,
-    }
-    print(json.dumps(event), flush=True)
-    _forward(event)
+def _log(event_type: str, severity: int = 6, **kwargs) -> None:
+    line = syslog_line(SERVICE_NAME, NODE_NAME, event_type, severity, **kwargs)
+    print(line, flush=True)
+    write_syslog_file(line)
+    forward_syslog(line, LOG_TARGET)
 
 
 def _parse_headers(msg: str) -> dict: