Add performance benchmarking for critical API endpoints

[Smartdevs17]

Description

No performance benchmarks exist, making it impossible to detect regressions. Implement automated benchmarking for critical API endpoints in CI.

Acceptance Criteria

• Benchmark suite for top 10 API endpoints
• k6 or autocannon integration
• CI gate that fails on >10% regression
• Historical benchmark trend dashboard
• Baseline establishment after optimization
• Load test scenarios for peak traffic simulation

Technical Scope

• backend/src/tests/
• .github/workflows/
• Edge: CI environment variability, benchmark flakiness handling