test: cover webhook queue boundaries

[eshulman2]

Summary

• atomically deduplicate delivery IDs and append webhook messages to Redis Streams
• wire Jira and GitHub webhook routes to report duplicate deliveries without requeueing
• add real FastAPI-to-Redis integration scenarios for accepted, duplicate, invalid-signature, and malformed deliveries
• add focused unit coverage for the atomic producer behavior

Why

Webhook deduplication utilities existed but were not connected to either route or the queue consumer, so repeated Jira or GitHub deliveries created duplicate stream messages. The Redis script makes delivery reservation and stream publication one atomic operation.

Verification

• 30 webhook route and queue producer unit tests passed
• 12 real-Redis webhook/queue integration tests passed
• full source Ruff lint and formatting checks passed

The local Python 3.14 environment stalled during the broader async fast suite; the required CI gate runs the supported Python 3.11 environment.