Phase 3: Domain config resolution logic

[gtema]

Scope

Implement the config resolution layer that merges sources with proper precedence.

Tasks

crates/core/ — Domain config resolution

• Build a config for a given domain:
  1. Start from global defaults
  2. Overlay file-based config (if domain_specific_drivers_enabled)
  3. Overlay DB-stored config (highest precedence)
• Config options:
  • domain_specific_drivers_enabled (bool, default false)
  • domain_configurations_from_database (bool, default true)
  • domain_config_dir (path, default /etc/keystone/domains)
• Option substitution resolution: expand %(option_name)s references before passing to drivers

Parent issue

Part of #954