feat: Implement backend query serialization

[ApostolisFalaras]

Description

Ensure requests sent to the single DBMS process are executed safely without overlapping stdin/stdout protocol messages.

Tasks

• Maintain a backend query queue
• Execute one DBMS request at a time when required by the protocol
• Associate responses with pending requests
• Reject requests after process failure
• Define maximum pending-query capacity
• Continue processing after recoverable query errors
• Prevent one failed query from corrupting later responses

Acceptance Criteria

• Concurrent HTTP requests cannot corrupt DBMS communication.
• Every response is delivered to the correct HTTP request.
• Query errors do not block subsequent queries.
• Process failure rejects outstanding requests cleanly.

Depends On

• feat: Implement DBMS request-response adapter