Implement a validator registry

As it stands, the [VALIDATORS dict](https://github.com/Groovbox/octofin/blob/master/src/config/validators.py#L15) is hardcoded, which limits modularity. If another developer wants to add custom settings and validators (e.g., from a plugin or extension), they'd need to modify the core code, which defeats modular design.

- [ ] Create a global validator registry with a simple API
- [ ] Make it so that extensions can use the `register_validator` decorator for their setting validation
- [ ] Make changes to the core validators to make use of the decorator as well.

Start with src/config/validators.py, especially the hardcoded VALIDATORS dict and its current validation flow. Define the registry API and register_validator decorator from the issue requirements, then update the core validators to use it. Done means extensions can register custom setting validators without modifying core code.

Written by the indexing model from the issue text.