feat(server): add async query execution support

[sivchari]

Summary

• Add support for async query execution with ?async=true query parameter
• Implement statement handle tracking with AsyncQueryState enum
• Add status polling via GET /api/v2/statements/{handle}
• Add query cancellation via POST /api/v2/statements/{handle}/cancel
• Add CancellationToken support for proper query cancellation

Changes

• server/Cargo.toml: Add parking_lot and tokio-util dependencies
• server/src/state.rs: Add AsyncQueryState, AsyncQueryEntry, and management methods
• server/src/handlers.rs: Implement async execution flow with tokio::spawn
• server/tests/integration_test.rs: Add tests for async query functionality

Test plan

• cargo test -p server passes
• cargo test --workspace passes
• cargo fmt --check passes