feat: add webhook registration system for creator trade events #425

[Chidubemkingsley]

• Add Webhook and WebhookEvent Prisma models with migration
• POST /creators/:id/webhooks — register webhook with wallet signature auth
• GET /creators/:id/webhooks — list active webhooks (creator auth)
• DELETE /creators/:id/webhooks/:webhook_id — remove a webhook
• Wallet signature verification via Stellar Ed25519 (x-wallet-address, x-signature, x-timestamp headers)
• Configurable max webhooks per creator (default 5)
• dispatchWebhookEvent() for use by indexer pipeline — fires POST to registered callback URLs with trade event payload
• Retry failed delivery up to 3 times with exponential backoff; flags webhook as failing after exhaustion
• events filter (buy/sell) limits which event types trigger the webhook
• @stellar/stellar-base dependency for Ed25519 key verification
• jest moduleNameMapper for chalk (ESM) to enable integration tests
• Unit tests: 15 passing (service + controllers)

Summary

• Task Completed
• closes Add trade webhook registration so creators are notified when their keys are bought or sold #425

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate when schema or generated types changed

Checklist

• Linked issue or backlog item
• No secrets or live credentials added
• Docs updated if setup or env changed
• Change is scoped to one problem