v3.0.0: authenticate with a yk_ App key, not email/app_password

The email+app_password -> /api/v1/auth/login bearer mint was retired
with personal app passwords (dns commit 834c90e). Switch to sending a
yeil App key (yk_<keyId>_<secret>) directly as the Bearer token, which
the DNS API's principal auth accepts. Single credential 'dns_yeil_api_key';
removed the login round-trip. BREAKING: existing credential files must
replace email/app_password with an api_key (an App with DNS record-write
permission, minted in team Apps). README + version bumped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

setup.py

@@ -2,7 +2,7 @@ from setuptools import setup, find_packages
 
 setup(
     name="certbot-dns-yeil",
-    version="2.0.0",
+    version="3.0.0",
     description="yeil DNS Authenticator plugin for Certbot",
     url="https://git.eskimo.dev/Yeil/certbot-dns-yeil",
     author="yeil",