Minor improvements to the API authentication

[ylembachar]

• 5. Replace the Retry-After value (e.g., 86,363 seconds) with a more human-readable format like "23 hours, 59 minutes".
• 6. Add a README with instructions on how to generate and manage API keys for internal use.
• 7. Redact or mask API keys in Caddy logs to prevent leaking them via logs
• 8. Add support for API key expiration, potentially via a flag or an expires_at field
• 9. Add unit tests for the key generation logic

[konradkonrad]

Some short notes for some of these points:

Replace the Retry-After value (e.g., 86,363 seconds) with a more human-readable format like "23 hours, 59 minutes".

Looks difficult from here -- value is generated by the plugin. Not sure, if Caddyfile allows calculation. So maybe an upstream feature request?

Redact or mask API keys in Caddy logs to prevent leaking them via logs

Removing this line should accomplish this.