fix(security): lock down signing-key file permissions

Private JWK files were written under the default umask (observed 0664 — group
and world readable). Create the key directory 0700, chmod private key files
(private_jwks.json, token_jwks.json) to 0600 after they are written, and
refuse to start if a pre-existing private key is group/world accessible.

Tests now use an isolated per-test key directory.

Refs: porchlight-91i

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_oidc/test_provider.py

@@ -1,6 +1,9 @@
+import os
 import shutil
 from pathlib import Path
 
+import pytest
+
 from porchlight.config import Settings
 from porchlight.oidc.claims import PorchlightUserInfo
 from porchlight.oidc.provider import create_oidc_server
@@ -53,3 +56,28 @@ def test_create_server_userinfo_is_porchlight() -> None:
         assert isinstance(server.context.userinfo, PorchlightUserInfo)
     finally:
         shutil.rmtree(key_path, ignore_errors=True)
+
+
+def test_signing_key_files_have_strict_permissions(tmp_path: Path) -> None:
+    key_path = tmp_path / "keys"
+    settings = Settings(issuer="http://localhost:8000", sqlite_path=":memory:", signing_key_path=str(key_path))
+    create_oidc_server(settings)
+
+    assert (key_path.stat().st_mode & 0o077) == 0, "key directory must not be group/world accessible"
+    for name in ("private_jwks.json", "token_jwks.json"):
+        f = key_path / name
+        assert f.exists()
+        assert (f.stat().st_mode & 0o077) == 0, f"{name} must be 0600"
+
+
+def test_startup_fails_on_world_readable_private_key(tmp_path: Path) -> None:
+    key_path = tmp_path / "keys"
+    key_path.mkdir()
+    # Simulate a pre-existing private key left group/world readable.
+    leaked = key_path / "private_jwks.json"
+    leaked.write_text("{}")
+    os.chmod(leaked, 0o644)
+
+    settings = Settings(issuer="http://localhost:8000", sqlite_path=":memory:", signing_key_path=str(key_path))
+    with pytest.raises(RuntimeError, match="permission"):
+        create_oidc_server(settings)