feat: metrics/health endpoint at GET /jobs/metrics.json

[eclectic-coding]

Summary

• Adds GET /jobs/metrics.json — a JSON-only endpoint for Prometheus scraping, uptime monitors, and external dashboards
• Response includes job counts (ready/scheduled/claimed/blocked/failed), throughput (completed_1h/24h), per-queue depth and paused state, and process health (total/healthy/stale/by_kind)
• slow_jobs count included when slow_job_threshold is configured
• Goes through the same authentication and connects_to middleware as all other routes — no special casing needed

Test plan

• 13 new request examples covering all response fields, slow_jobs conditional, healthy/stale process counts, and auth
• 234 examples, 0 failures, 99.79% line coverage
• bundle exec rake passes (RuboCop + RSpec)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0fa2db6) to head (e1910f0).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        26    +2     
  Lines          464       496   +32     
=========================================
+ Hits           464       496   +32     

Files with missing lines	Coverage Δ
.../controllers/solid_queue_web/metrics_controller.rb	100.00% <100.00%> (ø)
app/services/solid_queue_web/metrics_payload.rb	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.