feat(auth): add authentication with LDAP

[smayya337]

Proposed changes

• Create TJHSSTLDAPBackend, a new LDAP backend that authenticates against FreeIPA
• Remove PamAuthenticationBackend in favor of TJHSSTLDAPBackend
• Make TJHSSTLDAPBackend higher priority than MasterPasswordAuthenticationBackend in order to allow for auto-creation in all scenarios

Brief description of rationale

This will further streamline the account creation process by removing the need to create Ion accounts separately from FreeIPA accounts.

Please look over/test this carefully for security holes -- the whole thing was written by Claude Code as an experiment.

[smayya337]

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