fix: harden SSH auth, config validation, and startup probes

[aanogueira]

Accept SSH host keys in certificate_check callback (scoped to SSH only – HTTPS/TLS still validated via system CAs) so repos on hosts not in known_hosts can be fetched without manual intervention.

Add startupProbe (60 min window) to the Helm statefulset so large deployments with 1000+ repos survive initial indexing without the liveness probe killing the pod.

Harden config validation:

• Redact secrets in RepoConfig Debug impl to prevent log leakage
• Reject repo names with path-traversal sequences (../, leading /)
• Fail fast when auth_token is paired with an SSH URL (silent misconfiguration that caused the original indexing failure)