Add initial implementation of CalMiner with project structure, environment setup, and core features

- Create .env.example for environment variables
- Update README with project structure and development setup instructions
- Implement FastAPI application with API routes for scenarios and parameters
- Add database models for scenarios, parameters, and simulation results
- Introduce validation middleware for JSON requests
- Create services for running simulations and generating reports
- Add testing strategy and directory structure in documentation

.env.example

@@ -0,0 +1,4 @@
+# Example environment variables for CalMiner
+
+# PostgreSQL database connection URL
+database_url=postgresql://<user>:<password>@localhost:5432/calminer