feat: This Pull Request delivers the implementation of the production planning solution for the challenge.

[hagr27]

• Implemented the core logic to compute the optimal power distribution (merit order).
• Created a FastAPI application with a POST endpoint /productionplan.
• Defined models and services to calculate cost and available capacity for each plant.
• Added extensive unit and integration tests using pytest and FastAPI TestClient.
• Validated input schemas and handled unknown plant types with proper exceptions.
• Configured test structure and usage instructions.

For a detailed explanation of the implementation, please refer to the file README_hagr27.md.