TASK-015: Add validation for GlobalConfigUpdate.log_target and log_level

- Add LogLevel Literal type: CRITICAL, ERROR, WARNING, NOTICE, INFO, DEBUG
- Add log_target validation to accept special values (STDOUT, STDERR, SYSLOG)
  or validated file paths within allowed directories
- Update GlobalConfigResponse to use LogLevel type
- Add field_validator for log_target in both GlobalConfigUpdate and
  GlobalConfigResponse following the same pattern as AddLogPathRequest
- Add @autouse fixture to test_config_service.py to mock get_settings
- Update existing tests to use uppercase log level values
- Add 12 comprehensive tests for new validation in test_models.py
- Update Features.md to document valid log_target and log_level values
- Add section to Backend-Development.md documenting Literal types and
  field_validator patterns with examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_services/test_config_service.py

@@ -9,6 +9,7 @@ from unittest.mock import AsyncMock, patch
 
 import pytest
 
+from app.config import Settings
 from app.models.config import (
     GlobalConfigUpdate,
     JailConfigListResponse,
@@ -22,6 +23,25 @@ from app.services.config_service import (
     JailNotFoundError,
 )
 
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(autouse=True)
+def _mock_settings(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Mock get_settings for all tests in this module."""
+    def mock_get_settings() -> Settings:
+        return Settings(
+            database_path=":memory:",
+            fail2ban_socket="/tmp/fake.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="test-secret-key-do-not-use",
+        )
+
+    monkeypatch.setattr("app.models.config.get_settings", mock_get_settings)
+
+
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
@@ -472,7 +492,7 @@ class TestUpdateGlobalConfig:
             def __init__(self, **_kw: Any) -> None:
                 self.send = AsyncMock(side_effect=_send)
 
-        update = GlobalConfigUpdate(log_level="debug", db_purge_age=3600)
+        update = GlobalConfigUpdate(log_level="DEBUG", db_purge_age=3600)
         with patch("app.services.config_service.Fail2BanClient", _FakeClient):
             await config_service.update_global_config(_SOCKET, update)
 
@@ -492,7 +512,7 @@ class TestUpdateGlobalConfig:
             def __init__(self, **_kw: Any) -> None:
                 self.send = AsyncMock(side_effect=_send)
 
-        update = GlobalConfigUpdate(log_level="debug")
+        update = GlobalConfigUpdate(log_level="DEBUG")
         with patch("app.services.config_service.Fail2BanClient", _FakeClient):
             await config_service.update_global_config(_SOCKET, update)