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>

src/admin/guard.py

@@ -0,0 +1,12 @@
+import os
+
+from flask_login import current_user
+
+
+def is_admin() -> bool:
+    admin_email = os.getenv("ADMIN_EMAIL")
+    return bool(
+        admin_email
+        and current_user.is_authenticated
+        and current_user.email.lower() == admin_email.lower()
+    )