This pull request migrates the application's database from MongoDB to PostgreSQL. The transition is part of a broader effort to improve data integrity, performance, and maintainability. By leveraging PostgreSQL’s relational structure and robust feature set, we gain more control over data consistency, querying capabilities, and transactional operations.

• Support for transactions: Ensures data consistency across complex operations.
• Stronger data integrity: Enforces schemas, relationships, and constraints at the database level.
• Advanced querying capabilities: Enables powerful SQL queries and indexing strategies.
• Better alignment with RDF triple store: Simplifies integration and potential for joins where needed.
• Improved maintainability: Easier to manage migrations, structure, and long-term evolution.
• Operational maturity: PostgreSQL offers better tools for backup, monitoring, and scaling.