feat: metadata store (CN→email) and .ovpn config builder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_metadata.py

@@ -0,0 +1,16 @@
+from openvpncertupdate import load_metadata, save_email, get_email
+
+def test_load_empty(tmp_path):
+    assert load_metadata(str(tmp_path)) == {}
+
+def test_save_and_get(tmp_path):
+    save_email(str(tmp_path), "alice", "alice@example.com")
+    assert get_email(str(tmp_path), "alice") == "alice@example.com"
+
+def test_update_existing(tmp_path):
+    save_email(str(tmp_path), "alice", "old@example.com")
+    save_email(str(tmp_path), "alice", "new@example.com")
+    assert get_email(str(tmp_path), "alice") == "new@example.com"
+
+def test_missing_returns_empty(tmp_path):
+    assert get_email(str(tmp_path), "nobody") == ""