Fix key rotation bugs, support multi-user rotation, and improve logging/notifications

[haggaret]

• Fix 1Password SDK usage: use list() instead of nonexistent list_all()/.obj (project/plugins/1password.py)
• Fix SSH key loading to use paramiko's PKey.from_path for broader format support, with a fallback for RFC 5958 v2 Ed25519 keys that the bundled cryptography release fails to parse (project/plugins/ssh.py)
• Add HTTP status/body to Pingdom and Bitbucket API failure logs instead of swallowing the actual error (project/plugins/pingdom.py, bitbucket.py)
• Fix mail notifications: broken logo image, literal/unrendered newlines, unclickable URLs, and a stale office address (project/plugins/mail.py, notification-email.html)
• Name threads after the user/function they're running so crash tracebacks are attributable (project/utils.py)
• Support rotating/listing/validating multiple users via -u/--users (comma-separated), replacing the old singular --user flag; validate action no longer uses threading (project/main.py, README.md)
• Log which account/profile credentials were written to in parameter store, and clean up leftover debug print() statements (project/plugins/iam.py)