Fixes#74
## Why
`internal/proxy/circuit.go` implemented and tested a circuit breaker, but nothing ever called it — a repeatedly-failing upstream was still hit on every request.
## Changes
- Construct a `CircuitBreaker` in `NewEngine`.
- In `Engine.Fetch`: short-circuit when the breaker is open (serve stale from the store if present, otherwise return 503), `RecordFailure` on each `UpstreamError`, and `RecordSuccess` on a successful fetch.
## Validation
- `go test ./internal/proxy/` and `make e2e` pass.
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Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #90
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
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