Add configurable system and project run concurrency limits

[joyzoursky]

Summary

• Add configurable concurrency controls for test runs at system and project levels.
• Enforce a system-wide active run cap via environment variable.
• Add per-project max concurrent run setting with a default of 1 and configurable ceiling.

Changes

• Schema/API
• Add Project.maxConcurrentRuns (default 1) to Prisma schema.
• Add env configs:
• RUNNER_MAX_CONCURRENT_RUNS (default 10)
• PROJECT_MAX_CONCURRENT_RUNS_MAX (default 5)
• Extend project APIs to return and validate maxConcurrentRuns and maxConcurrentRunsLimit.
• Scheduler/dispatch
• Enforce both system and project concurrency limits in Android claim path.
• Enforce both limits in browser dispatch path.
• Add queued browser run backfill dispatch when slots are freed (complete/fail/cancel/lease-reaper/MCP cancellation).
• UI
• Add Settings tab on project page after Variables.
• Add project settings form to edit max concurrent runs.
• Add i18n strings for EN / zh-Hant / zh-Hans.
• Docs/config
• Update .env.example and k8s deploy docs for new env vars.

Validation

• Lint/Type-check: npm run lint (pass)
• Verify: npm run verify (pass)
• Tests:
• npm test -- src/lib/runners/__tests__/claim-service.test.ts src/app/api/run-test/route.test.ts src/lib/mcp/__tests__/run-execution.test.ts 'src/app/api/test-runs/[id]/cancel/route.test.ts' src/lib/runtime/__tests__/local-browser-runner.usage.test.ts src/lib/mcp/__tests__/run-cancellation.test.ts (pass)

Breaking Changes

• Database schema change: adds Project.maxConcurrentRuns.
• New runtime env vars introduced (with defaults).

Risks

• Dispatch behavior now triggers queued browser run checks at more transition points; this increases claim attempts under high churn.

Follow-ups

• Run npx prisma db push (or migration flow) in target environments before deploy.
• Monitor queue throughput and tune RUNNER_MAX_CONCURRENT_RUNS / PROJECT_MAX_CONCURRENT_RUNS_MAX per environment.