feat(matches): backoff and circuit breaker for the external result feed

[bilhokista]

Closes #1620.

What this changes

HttpExternalResultFeedClient.fetchResults() was a bare firstValueFrom(httpService.get(...)) with no error handling: a single upstream blip failed a poll, and a sustained outage meant every scheduled poll kept hammering a service that was already down.

Backoff reuses the existing withRetry / computeBackoffDelay in backend/src/common/retry.util.ts rather than introducing a second backoff implementation, so the feed behaves like soroban.service.ts and notifications/email.service.ts already do. The issue describes the client as having "no backoff", which is accurate — but the helper it needs was already in the codebase and simply not wired in.

Circuit breaker is new: backend/src/common/circuit-breaker.ts, placed alongside retry.util.ts. Retry handles one bad call; the breaker handles a bad upstream. It is deliberately small — one upstream, one instance, constructed by the client that owns the call — rather than a decorator or a module.

• closed → calls pass through, consecutive failures counted
• open → calls rejected locally with CircuitOpenError for openMs
• half-open → one probe admitted; success closes, failure re-opens for a full window

Retry sits inside the breaker, so one poll gets one retry budget and the breaker counts polls rather than individual attempts.

Error classification (isTransientFeedError): 408/425/429/5xx and requests that never got a response are retried; other 4xx are not, because a wrong URL or a rejected credential will be rejected identically on every attempt.

Health signal is exposed as client.getHealth(): BreakerHealth — { state, consecutiveFailures, retryAfterMs, lastError }.

One scope decision worth flagging

The issue lists backend/src/health in its file list. I did not wire the feed into HealthService: that service composes injected Terminus indicators, and adding the feed changes which dependencies /health reports on — an architecture call for a maintainer, not a side effect of this PR. The signal is exposed and ready to consume; I'm happy to add the indicator in this PR if you'd like it here, and there is a comment in the client saying so.

Tests

backend/src/common/circuit-breaker.spec.ts (new, 6 cases) drives the breaker through an injected clock, so no test sleeps through a real openMs:

• passes results through while closed
• opens after the failure threshold
• rejects without calling upstream while open
• half-open probe recovers and resets the failure count
• failed probe re-opens for a full window
• only one probe is admitted while half-open

external-result-feed.client.spec.ts adds status-classification cases, "retries a transient failure", "does not retry a rejected credential", "opens after repeated failures and stops calling the feed", and "closes again when the half-open probe succeeds". The existing URL/credential test is unchanged and still passes.

Honest note on verification: I could not run the backend Jest suite locally (no full workspace install). I did two things instead: every changed file was parsed with the TypeScript compiler API, and the breaker's state machine was transpiled and executed standalone against the same 14 assertions the spec makes — all passed. CI is the real check on the Nest-level tests.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CrfEY1tvXrbeMDAUzxfuk7

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Project	Deployment	Actions	Updated
insight-arena-4rll	Ready	Preview	Sep 10, 2026 1:26pm UTC