test: add async backpressure integration coverage

[darthfork]

Summary

• add HTTP-level integration coverage for async queue saturation and queue-full drop metrics
• cover async processor panic recovery and worker continuation from observable /metrics output
• cover graceful processor drain behavior for accepted queued events
• document async worker/queue configuration, backpressure responses, failure metrics, and drain behavior

Closes #55

Test plan

• make unit-test
• make integration-test
• PATH="$(go env GOPATH)/bin:$PATH" make lint