Abstract Database layer to support external persistence (Postgres/Redis) #21

The engine currently relies on local SQLite for session and state persistence. This limits horizontal scalability in distributed backend architectures where multiple engine instances may need to share state.

Requirements:

• Abstract the Database layer to allow pluggable storage backends.
• Implement support for Postgres (for shared relational state) and Redis (for distributed locking/caching).
• Ensure backward compatibility with existing SQLite migrations.