Implement REST API v1 — signal endpoint for external events

[treblereel]

Context

Part of epic #1 — Build production-ready REST microservice with k8s deployment on top of casehub-engine. This provides the mechanism for external systems to send events/signals into running cases to trigger state transitions.

What

Implement signal injection endpoint:

• POST /api/v1/cases/{caseId}/signals — send external event/signal into case with payload { signalName, data }

Acceptance Criteria

• POST endpoint accepts signal name and optional data payload
• Signal is routed to casehub-engine's signal processing mechanism
• Returns 202 Accepted if signal queued successfully
• Returns 404 if caseId doesn't exist
• Returns 400 if signal payload validation fails
• Idempotency: duplicate signals within idempotency window are deduplicated
• Error responses follow RFC 7807 format
• Integration tests verify signal triggers worker execution and context updates

Notes

• Leverage casehub-engine's signal/event dispatching mechanism
• Check if casehub-engine has built-in signal deduplication (DLQ module from #194, #193)
• Consider adding signal ID for client-side idempotency tracking
• Response should include signal acceptance confirmation, not execution result (async model)
• File paths: casehub-engine's signal handling, WorkerScheduleEventHandler