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>

decnet/logging/syslog_formatter.py

@@ -0,0 +1,84 @@
+"""
+RFC 5424 syslog formatter for DECNET.
+
+Produces fully-compliant syslog messages:
+  <PRI>1 TIMESTAMP HOSTNAME APP-NAME PROCID MSGID [SD-ELEMENT] MSG
+
+Facility: local0 (16)
+PEN for structured data: decnet@55555
+"""
+
+from datetime import datetime, timezone
+from typing import Any
+
+
+FACILITY_LOCAL0 = 16
+NILVALUE = "-"
+_SD_ID = "decnet@55555"
+
+SEVERITY_INFO    = 6
+SEVERITY_WARNING = 4
+SEVERITY_ERROR   = 3
+
+# RFC 5424 field length limits
+_MAX_HOSTNAME = 255
+_MAX_APPNAME  = 48
+_MAX_MSGID    = 32
+
+
+def _pri(severity: int) -> str:
+    return f"<{FACILITY_LOCAL0 * 8 + severity}>"
+
+
+def _truncate(value: str, maxlen: int) -> str:
+    return value[:maxlen] if len(value) > maxlen else value
+
+
+def _sd_escape(value: str) -> str:
+    """Escape SD-PARAM-VALUE per RFC 5424 §6.3.3."""
+    return value.replace("\\", "\\\\").replace('"', '\\"').replace("]", "\\]")
+
+
+def _sd_element(fields: dict[str, Any]) -> str:
+    if not fields:
+        return NILVALUE
+    params = " ".join(
+        f'{k}="{_sd_escape(str(v))}"'
+        for k, v in fields.items()
+    )
+    return f"[{_SD_ID} {params}]"
+
+
+def format_rfc5424(
+    service: str,
+    hostname: str,
+    event_type: str,
+    severity: int = SEVERITY_INFO,
+    timestamp: datetime | None = None,
+    msg: str | None = None,
+    **fields: Any,
+) -> str:
+    """
+    Return a single RFC 5424-compliant syslog line (no trailing newline).
+
+    Args:
+        service:    APP-NAME field (e.g. "http", "mysql")
+        hostname:   HOSTNAME field (the decky node name)
+        event_type: MSGID field (e.g. "request", "login_attempt")
+        severity:   Syslog severity integer (default: INFO=6)
+        timestamp:  Datetime to use; defaults to utcnow
+        msg:        Optional free-text MSG suffix
+        **fields:   Arbitrary key=value pairs encoded in structured data
+    """
+    ts = (timestamp or datetime.now(timezone.utc)).isoformat()
+
+    pri       = _pri(severity)
+    version   = "1"
+    host      = _truncate(hostname or NILVALUE, _MAX_HOSTNAME)
+    appname   = _truncate(service or NILVALUE, _MAX_APPNAME)
+    procid    = NILVALUE
+    msgid     = _truncate(event_type or NILVALUE, _MAX_MSGID)
+    sd        = _sd_element(fields)
+    message   = f" {msg}" if msg else ""
+
+    return f"{pri}{version} {ts} {host} {appname} {procid} {msgid} {sd}{message}"