Deduplicate retried GitHub webhook deliveries

[darthfork]

Parent issue: #35

Summary

GitHub may retry webhook deliveries, and today those retries will be processed as new events. Add deduplication keyed by X-GitHub-Delivery with a bounded retention strategy.

Deliverables

• Delivery ID cache with TTL.
• Metrics for duplicate deliveries seen/dropped.
• Tests covering duplicate processing behavior.

Acceptance criteria

• Replayed deliveries do not inflate counters or disturb gauge/state handling.