fix: update Cryptgeon API payload to match newer server schema

Newer Cryptgeon versions expect `contents` as a base64 string (not a byte
array) and require a `meta` string field.

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

openvpncertupdate.py

@@ -319,7 +319,8 @@ def create_note(content: str, base_url: str) -> str:
     ciphertext = aesgcm.encrypt(nonce, content.encode("utf-8"), None)
 
     payload = json.dumps({
-        "contents": list(nonce + ciphertext),
+        "contents": base64.b64encode(nonce + ciphertext).decode("ascii"),
+        "meta": "",
         "views": 1,
         "type": "text",
     }).encode("utf-8")