feat(config): wrap credentials in SecretStr (audit S-4)

database_url, migration_database_url, mcp_auth_token and the mathpix app
id/key are now pydantic SecretStr, so they render as '**********' in any
repr/log/traceback instead of leaking the plaintext credential. Consumers
(db.get_conn, cli.migrate, mcp._require_http_token, mathpix.extract_formulas)
call .get_secret_value() at the point of use. Tests updated to the SecretStr
type.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/parsing/test_mathpix.py

@@ -10,6 +10,7 @@ from typing import Any
 from unittest.mock import MagicMock, patch
 
 import pytest
+from pydantic import SecretStr
 
 from codex.models import FormulaChunk
 
@@ -295,8 +296,8 @@ class TestRouteSelection:
     def test_mathpix_selected_when_creds_present(self) -> None:
         """extract_formulas calls MathPix backend when both creds are set."""
         mock_settings = MagicMock()
-        mock_settings.mathpix_app_id = "my_id"
-        mock_settings.mathpix_app_key = "my_key"
+        mock_settings.mathpix_app_id = SecretStr("my_id")
+        mock_settings.mathpix_app_key = SecretStr("my_key")
         mock_settings.pix2tex_fallback = True
 
         with (