Add settings management in conf module

[AnthonyTedde]

Feature summary

Create a single, project-wide settings object that every module can import and read from.

What to build

1. Settings loader

   • Pull values from a user-supplied settings file and fall back to default values when missing.
   • The user file is chosen via an environment variable.

2. Lazy wrapper

   • Don’t load the settings until they’re actually requested.

3. Singleton behaviour

   • Once loaded, the same settings instance is shared everywhere and cannot be modified.

4. Package entry-point

   • Expose the ready-to-use object as from myproject.conf import settings.

5. Basic tests & docs

   • Verify correct merge order, lazy loading, immutability, and the “missing env-var” error.