Create webhook endpoint for Stellar Horizon event notifications

[Anuoluwapo25]

Summary

The event indexer currently polls Stellar Horizon on a timer. Supporting push webhooks from Horizon (or a relay service) would reduce polling overhead and give near-real-time event processing.

Proposed Endpoint

POST /api/webhooks/horizon

• Verifies HMAC signature of incoming payload
• Parses event and dispatches to the existing event-indexer service
• Returns 200 immediately, processes async

Acceptance Criteria

• Endpoint created with HMAC-SHA256 signature verification
• Unverified requests rejected with 401
• Payload parsed and routed to event-indexer.service.ts
• Integration test with a signed and unsigned payload
• WEBHOOK_SECRET added to server/.env.example

Files

• server/src/routes/
• server/src/services/event-indexer.service.ts