Config/CLI: Rename `--config-file` to `--defaults-yaml` and improve command help

[lastzero]

As a CLI user, the use of the flags is clearer if --config-file is renamed to --defaults-yaml, as I would otherwise expect it to be possible to set all configuration values in the YAML file passed as an argument.

If you pass a YAML file via --defaults-yaml/--config-file`, the values are only used to set defaults. These can still be overridden by CLI parameters and environment variables (which are always present in most Docker/Cloud environments, so you can't really change anything).

In case you want PhotoPrism to ignore the environment and prefer values from a configuration file instead, you should use the --config-path flag / the PHOTOPRISM_CONFIG_PATH variable:

• Values in [config-path]/options.yml always take precedence over CLI flags and environment variables if present
• Options changed in the advanced settings UI are automatically stored here
• To avoid write conflicts, it is best to modify the options.yml file only when the backend is not running
• Command help has been updated to explain which flag does what (which wasn't really the case before)