feat(admin): add is_admin() access guard

Implement access control function for admin panel. Returns True only if
ADMIN_EMAIL env var is set, user is authenticated, and email matches
case-insensitively.

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

tests/admin/test_guard.py

@@ -0,0 +1,42 @@
+from unittest.mock import patch
+
+from src.admin import guard
+
+
+def _fake_user(email: str, authenticated: bool = True):
+    user = type("U", (), {})()
+    user.is_authenticated = authenticated
+    user.email = email
+    return user
+
+
+def test_is_admin_true_for_matching_email(monkeypatch):
+    monkeypatch.setenv("ADMIN_EMAIL", "admin@ex.fr")
+    with patch("src.admin.guard.current_user", _fake_user("admin@ex.fr")):
+        assert guard.is_admin() is True
+
+
+def test_is_admin_true_case_insensitive(monkeypatch):
+    monkeypatch.setenv("ADMIN_EMAIL", "Admin@Ex.fr")
+    with patch("src.admin.guard.current_user", _fake_user("admin@ex.fr")):
+        assert guard.is_admin() is True
+
+
+def test_is_admin_false_for_different_email(monkeypatch):
+    monkeypatch.setenv("ADMIN_EMAIL", "admin@ex.fr")
+    with patch("src.admin.guard.current_user", _fake_user("someone@ex.fr")):
+        assert guard.is_admin() is False
+
+
+def test_is_admin_false_for_anonymous(monkeypatch):
+    monkeypatch.setenv("ADMIN_EMAIL", "admin@ex.fr")
+    with patch(
+        "src.admin.guard.current_user", _fake_user("admin@ex.fr", authenticated=False)
+    ):
+        assert guard.is_admin() is False
+
+
+def test_is_admin_false_when_admin_email_unset(monkeypatch):
+    monkeypatch.delenv("ADMIN_EMAIL", raising=False)
+    with patch("src.admin.guard.current_user", _fake_user("admin@ex.fr")):
+        assert guard.is_admin() is False