[Feature] Make importer settings configurable via YAML

[TheBoatyMcBoatFace]

Overview

Currently settings.SOURCES and API endpoints are hard-coded in settings.py. Allow users to override or add sources via a config.yml file (listing name, importer_class, api_url, etc.).

Background

Config-driven design empowers users to add new data sources without touching code.

References

• PyYAML Usage
• Dynamic Imports in Python

Acceptance Criteria

• Create config.yml schema for sources (in project root).
• Modify settings.py to load from config.yml if it exists, fallback to defaults.
• Document the new config file format in README.
• Add tests that simulate a custom config with a dummy importer.