Add support for backup LDAP server

[ap0int3d]

Hi,

Without entering in too much details, in an AD environment, Domain Controllers obtain their certificate from ADCS with a predefined template in which it is not possible to add a SAN like "ldap.example.com". This makes it difficult to create a single DNS entry with round robin between Domain Controllers and hence makes it impossible to use LDAPS or Start TLS in WebSSH with a single redundant configuration entry.

That's why I think it could be useful to add support for a backup LDAP server in the compose-ldap file.

[bifrost0x]

Implemented in PR #189. It adds an optional LDAP_BACKUP_URL for a second server in the same logical directory. WebSSH fails over only on transport or availability errors during user lookup, password bind, and readiness checks; a completed bind that rejects the password is authoritative and is not retried. The endpoint that resolved the user is preferred for the following password bind. Both URLs are validated as distinct LDAP/LDAPS endpoints, each direct hostname must match its TLS certificate, and the deployment guide, Compose example, wiki, and regression coverage have been updated.

[ap0int3d]

Thanks a lot for adding support. I will try and test that as soon as possible.