feat(config): add LOG_LEVEL setting and configure root logging at startup

- Add `log_level` setting to config.py (default: INFO, env: LOG_LEVEL)
- Configure Python root logger in main.py with standard precedence:
  LOG_LEVEL explicit > DEBUG=true implies DEBUG > default INFO
- Add timestamp to log format for production readability
- Suppress noisy third-party loggers at DEBUG level
- Add comprehensive debug logging to all auth functions
- Add LOG_LEVEL/DEBUG to .env.demo and ConfigurationGuide.md
- Add tests for logging config and auth debug output

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

tests/test_auth.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import hashlib
+import logging
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
@@ -38,6 +39,40 @@ class TestGetCurrentUser:
         result = get_current_user(mock_request)
         assert result is None
 
+    def test_logs_debug_when_session_user_found(self, caplog):
+        """Test that get_current_user emits a DEBUG log when session user is found."""
+        mock_request = MagicMock(spec=Request)
+        mock_request.session = {"user": {"id": "u1", "preferred_username": "alice"}}
+        mock_request.state = MagicMock(spec=[])  # no api_token_user attribute
+
+        with caplog.at_level(logging.DEBUG, logger="app.auth"):
+            get_current_user(mock_request)
+
+        assert any("[AUTH] get_current_user: resolved from session" in m for m in caplog.messages)
+
+    def test_logs_debug_when_no_user(self, caplog):
+        """Test that get_current_user emits a DEBUG log when no user is present."""
+        mock_request = MagicMock(spec=Request)
+        mock_request.session = {}
+        mock_request.state = MagicMock(spec=[])
+
+        with caplog.at_level(logging.DEBUG, logger="app.auth"):
+            get_current_user(mock_request)
+
+        assert any("[AUTH] get_current_user: no user in session or API token" in m for m in caplog.messages)
+
+    def test_logs_debug_when_api_token_user(self, caplog):
+        """Test that get_current_user emits a DEBUG log when resolved from API token."""
+        mock_request = MagicMock(spec=Request)
+        mock_request.state.api_token_user = {"id": "tok_user"}
+        mock_request.session = {}
+
+        with caplog.at_level(logging.DEBUG, logger="app.auth"):
+            result = get_current_user(mock_request)
+
+        assert result == {"id": "tok_user"}
+        assert any("[AUTH] get_current_user: resolved from API token" in m for m in caplog.messages)
+
 
 @pytest.mark.unit
 class TestGetGravatarUrl: