Add Performance Benchmarks

Create benchmarks to track server performance over time.

Description: Having benchmarks would help identify performance regressions and opportunities for optimization.

Tasks:

• Create benchmark suite for query throughput
• Add benchmarks for connection handling
• Add benchmarks for common query patterns
• Create automated benchmark CI job
• Add benchmark results to documentation
• Document how to run benchmarks locally

Related files:

• benchmarks/ directory (new)