Issuer key auto-update fails to register keys present before startup

[h2zh]

The integration team found two gaps in the issuer-key auto-update mechanism that leave the registry's stored JWKS out of sync with the keys an origin actually holds:

1. Keys present at startup are never advertised. If a second key file already exists in the issuer-keys directory before the Pelican process starts, only the first (active) key gets registered. The second key is never pushed to the registry.
2. Issuer key auto-update never runs on Caches #3505