Add Gold layer analysis, visualization, notebook, and code cleanup

This Merge Request integrates all the work completed on the Gold layer of the project, including analysis, visualization tools, a dedicated notebook, and final code cleanup.

Added Features

Gold Layer

• Added gold_analysis.py with complete transformation logic.
• Added gold_visualization.py including plotting functions for key metrics.
• Implemented handling of incomplete current-year data in Gold transformations.

Notebook

• Created a dedicated Gold notebook for interactive visualization and exploration.

Refactoring

• Cleaned api_rte.py, commented lines and improving readability.

How to Test

1. Run Bronze, Silver, and Gold pipeline (as already validated in the previous Merge Request)
2. Open the Gold notebook in notebooks/01_gold_visualization.ipynb
3. Verify that plots and transformations reflect updated data.
4. Confirm no errors in API module.