feat: Implement backend query timeout handling

[ApostolisFalaras]

Description

Prevent indefinitely running DBMS operations from leaving backend requests unresolved.

Tasks

• Define a query timeout
• Track query execution duration
• Reject timed-out HTTP requests
• Define DBMS cancellation behavior
• Handle an unresponsive DBMS process
• Ensure the request queue remains consistent after timeout

Acceptance Criteria

• Queries cannot leave HTTP requests pending indefinitely.
• Timeout errors use a consistent response format.
• Later queries remain usable after recoverable timeout scenarios.

Depends On

• feat: Implement backend query serialization