Generic Backend Service Rules for Sourcegraph Cody

# Generic Backend Service Rules for Sourcegraph Cody

## Service design
- Clear boundary: handlers/controllers, domain services, repositories.
- Keep business logic out of transport layer.
- Domain errors mapped explicitly to response errors.

## Resilience
- Timeouts + retries + circuit breaker for external dependencies.
- Queue heavy/slow side effects off request path.
- Graceful shutdown with in-flight request draining.

## Operability
- Health/readiness checks must reflect real dependency state.
- Backpressure strategy for spikes (rate limit, queue, degrade gracefully).