Skip security tests when webapp deps are not installed

CI only installs base dependencies; importing ark.webapp.crypto
triggers fastapi import via ark/webapp/__init__.py. Add
pytest.importorskip guards so the test file is skipped gracefully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JihaoXin

tests/test_security.py

@@ -5,6 +5,9 @@
 import tempfile
 from pathlib import Path
 
+pytest.importorskip("cryptography", reason="webapp optional deps not installed")
+pytest.importorskip("fastapi", reason="webapp optional deps not installed")
+
 from ark.webapp.crypto import encrypt_text, decrypt_text
 
 class TestCredentialSecurity: