Improve load testing: replace/repair loadtest.sh with a reliable harness

[aplgr]

Goal

Provide an easy, reliable way to run load tests locally against the stack.

Why

The service involves rate limiting + auth + Chrome rendering. Load tests are the fastest way to surface concurrency issues, leaks, and throttling behavior.

Scope

• Keep scripts/loadtest.sh as entrypoint, but make it robust:
  • config via env/flags: base URL, duration, rate, concurrency, endpoint (HTML vs URL), output report file
  • optional API key / token input (separate token for higher limits)
  • handle 429 gracefully (count + backoff) instead of failing immediately
  • print summary: total requests, success rate, 4xx/5xx breakdown, p50/p95 latency (if tool supports)
• Recommended: use a standard tool (Vegeta / hey / k6), either:
  • bundled via Docker image, or
  • auto-detect installed binary and fallback to curl-loop.

Acceptance criteria

• ./scripts/loadtest.sh runs end-to-end on a fresh clone (after make start)
• Produces a deterministic, readable summary
• Works in both “public/no-key” mode and “API-key” mode

Notes / hints

Document recommended test token settings so contributors don’t accidentally DDoS the service with public limits.