Wrap VercelService API calls with CircuitBreaker to prevent cascade failures

[temma02]

Description

CircuitBreaker is implemented and tested but VercelService calls the Vercel API directly without any circuit-breaking. A Vercel outage causes all deployment requests to hang until timeout, blocking the entire pipeline queue.

Requirements and context

• Must be secure, tested, and documented where applicable
• Should stay reviewable and fit the current monorepo structure
• Relevant files: apps/backend/src/lib/api/circuit-breaker.ts, apps/backend/src/services/vercel.service.ts, apps/backend/tests/chaos/external-services.chaos.test.ts

Suggested execution

• Create branch: issue-019-circuit-breaker-vercel-api-calls
• Keep changes scoped to the issue and reference the task IDs in the PR

Implement changes

• Inject a CircuitBreaker instance into VercelService constructor
• Wrap request() with breaker.call()
• Expose circuit state in the health-check cron response so ops can observe it

Test and commit

• Extend external-services.chaos.test.ts with Vercel-specific open/half-open/closed transitions
• Verify CircuitOpenError is surfaced as a retryable pipeline error
• Edge case: circuit opens mid-deployment — pipeline should fail fast, not hang

Example commit message

feat(vercel): wrap API calls with circuit breaker

Guidelines

• Prefer small, reviewable PRs
• Keep naming and data contracts consistent with the spec docs

[Flamki]

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