Add async ingestion and backpressure integration coverage

[darthfork]

Parent issue: #52

Summary

Add integration tests for the async event processor so queueing, overload behavior, and eventual metric updates are validated under realistic request flow.

Scope

• Exercise async webhook ingestion over HTTP.
• Validate queue success and queue-full behavior.
• Validate dropped-event and processing metrics.
• Validate shutdown behavior with in-flight work where feasible.

Scenarios

• Accepted events are eventually reflected in /metrics.
• Queue saturation returns the expected non-success response.
• Dropped event counters increment when the queue is full.
• Processing failure metrics increment on handler panic/failure paths.
• Shutdown behavior is documented and covered by at least one test.

Design notes

• Use bounded waiting/polling to avoid flaky timing assumptions.
• Keep deterministic control over worker count and queue size.
• Focus on observable behavior, not goroutine internals.

Acceptance criteria

• Async behavior is covered through request/metrics observation.
• Backpressure and error visibility are validated.
• Tests are reliable in CI.