feat(config): add JSON structured logging and syslog forwarding for application logs

- Add LOG_FORMAT setting (text/json) for structured JSON log output
- Add LOG_SYSLOG_* settings for direct syslog forwarding of app logs
- JSON format compatible with Grafana Loki, Splunk, ELK, Datadog
- Syslog forwarding uses Python's SysLogHandler (UDP/TCP)
- Update .env.demo and ConfigurationGuide.md with all new settings
- Add tests for JSON formatter and syslog config fields

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_logging_config.py

@@ -75,8 +75,6 @@ class TestEffectiveLogLevel:
             env = os.environ.copy()
             env.pop("LOG_LEVEL", None)
             with patch.dict(os.environ, env, clear=True):
-                from app.config import Settings as S
-
                 s = Settings(
                     database_url="sqlite:///test.db",
                     redis_url="redis://localhost:6379",
@@ -100,8 +98,6 @@ class TestEffectiveLogLevel:
     def test_explicit_log_level_overrides_debug(self):
         """When LOG_LEVEL is explicitly set, it takes precedence over DEBUG=True."""
         with patch.dict(os.environ, {"LOG_LEVEL": "WARNING", "DEBUG": "true"}, clear=False):
-            from app.config import Settings as S
-
             s = Settings(
                 database_url="sqlite:///test.db",
                 redis_url="redis://localhost:6379",
@@ -171,3 +167,119 @@ class TestLoggingConfiguredAtStartup:
         root = logging.getLogger()
         # The test env doesn't set DEBUG=True, so the level should be INFO (20)
         assert root.level <= logging.INFO
+
+
+@pytest.mark.unit
+class TestJsonFormatter:
+    """Tests for the _JsonFormatter used when LOG_FORMAT=json."""
+
+    def _make_formatter(self):
+        """Lazily import the JSON formatter from main module."""
+        from app.main import _JsonFormatter
+
+        return _JsonFormatter()
+
+    def test_output_is_valid_json(self):
+        """JSON formatter output should be parseable JSON."""
+        import json
+
+        fmt = self._make_formatter()
+        record = logging.LogRecord(
+            name="test.logger",
+            level=logging.INFO,
+            pathname="test.py",
+            lineno=42,
+            msg="Hello %s",
+            args=("world",),
+            exc_info=None,
+        )
+        result = fmt.format(record)
+        parsed = json.loads(result)
+        assert parsed["level"] == "INFO"
+        assert parsed["logger"] == "test.logger"
+        assert parsed["message"] == "Hello world"
+        assert parsed["lineno"] == 42
+
+    def test_includes_timestamp_iso8601(self):
+        """JSON output should contain an ISO 8601 timestamp."""
+        import json
+
+        fmt = self._make_formatter()
+        record = logging.LogRecord(
+            name="x",
+            level=logging.DEBUG,
+            pathname="x.py",
+            lineno=1,
+            msg="test",
+            args=(),
+            exc_info=None,
+        )
+        parsed = json.loads(fmt.format(record))
+        assert "timestamp" in parsed
+        # ISO 8601 timestamps contain "T" and "+00:00" (UTC)
+        assert "T" in parsed["timestamp"]
+
+    def test_includes_exc_info_when_present(self):
+        """JSON output should include exc_info when an exception is logged."""
+        import json
+
+        fmt = self._make_formatter()
+        try:
+            raise ValueError("boom")  # noqa: TRY301
+        except ValueError:
+            import sys
+
+            record = logging.LogRecord(
+                name="x",
+                level=logging.ERROR,
+                pathname="x.py",
+                lineno=1,
+                msg="error",
+                args=(),
+                exc_info=sys.exc_info(),
+            )
+        parsed = json.loads(fmt.format(record))
+        assert "exc_info" in parsed
+        assert "ValueError" in parsed["exc_info"]
+
+
+@pytest.mark.unit
+class TestLogFormatSetting:
+    """Tests for the log_format and log_syslog_* config fields."""
+
+    _BASE_KWARGS = {
+        "database_url": "sqlite:///test.db",
+        "redis_url": "redis://localhost:6379",
+        "openai_api_key": "test",
+        "azure_ai_key": "test",
+        "azure_region": "test",
+        "azure_endpoint": "https://test.example.com",
+        "gotenberg_url": "http://localhost:3000",
+        "workdir": "/tmp",
+        "auth_enabled": False,
+        "session_secret": None,
+    }
+
+    def test_log_format_default_is_text(self):
+        """Test that log_format defaults to 'text'."""
+        config = Settings(**self._BASE_KWARGS)
+        assert config.log_format == "text"
+
+    def test_log_format_accepts_json(self):
+        """Test that log_format accepts 'json'."""
+        config = Settings(**self._BASE_KWARGS, log_format="json")
+        assert config.log_format == "json"
+
+    def test_log_syslog_defaults(self):
+        """Test syslog forwarding defaults."""
+        config = Settings(**self._BASE_KWARGS)
+        assert config.log_syslog_enabled is False
+        assert config.log_syslog_host == "localhost"
+        assert config.log_syslog_port == 514
+        assert config.log_syslog_protocol == "udp"
+
+    def test_log_syslog_can_be_enabled(self):
+        """Test that syslog forwarding can be enabled."""
+        config = Settings(**self._BASE_KWARGS, log_syslog_enabled=True, log_syslog_host="syslog.example.com")
+        assert config.log_syslog_enabled is True
+        assert config.log_syslog_host == "syslog.example.com"