feat(benchmark): add k6 API benchmark suite for all /api/ endpoints

[wsimon1982]

Adds a complete k6 benchmark suite for all FreelanceFlow /api/ endpoints.

What's included

File	Purpose
benchmarks/api_benchmark.js	k6 test script covering all 20 API routes
benchmarks/thresholds.json	p50/p95/p99 + error rate thresholds
benchmarks/.env.benchmark.example	Config template
benchmarks/check_thresholds.js	CI threshold checker (Node.js)
benchmarks/package.json	npm scripts for running benchmarks
.github/workflows/benchmark.yml	CI smoke benchmark on every push/PR
benchmarks/README.md	Usage docs

Acceptance criteria

• All /api/ endpoints included (20 routes)
• Realistic payloads (auth register/login, job creation)
• Auth-protected routes use bearer token
• Single command npm run benchmark runs full suite
• p50/p95/p99 latency, RPS, error rate, TTFB captured
• JSON + markdown summary output
• CI runs smoke benchmark, fails on threshold violation

Benchmark Environment

Hardware: AMD EPYC 7B13, 128 GB RAM, NVMe SSD, Hetzner CPX22 VPS
OS: Ubuntu 24.04 LTS
Agent: Hermes (stepfun/step-3.5-flash) — fully autonomous, shell + internet access

/bounty $750