PostgreSQL support

[dcorral]

Currently, the RLN node uses sea-orm as the ORM, but only SQLite has been tested and used in practice. To prepare for production or more demanding environments, we need to ensure PostgreSQL is fully supported, tested, and documented.

Goal

Implement and test PostgreSQL as an alternative database backend for the RLN node.

Tasks

• Verify existing sea-orm entity definitions are compatible with PostgreSQL
• Update database connection logic to support PostgreSQL (e.g., connection string format, pool configuration)
• Add PostgreSQL-specific migration handling (if needed)
• Write or adapt integration tests that run against PostgreSQL
• Ensure all CRUD operations and RLN-specific queries work correctly
• Document how to run the node with PostgreSQL (environment variables, config, etc.)
• Add CI pipeline step to test against PostgreSQL