Config.DefaultsYaml() resolves the default options YAML file. When
PHOTOPRISM_DEFAULTS_YAML points to a readable file it will be used;
otherwise it falls back to `defaults.{yml,yaml}` inside the active
config directory.
This allows instances without `/etc/photoprism/defaults.yml` to
still load local defaults, e.g., in containerized environments.
Signed-off-by: Michael Mayer <michael@photoprism.app>
