Implement outbound webhook delivery with retries and dead-letter handling

[bridgetthnkechi87-cloud]

Closes #295

Summary

• Added outbound webhook event types and dispatch support in packages/webhook-utils
• Reused the existing HMAC signing/verification flow for symmetric webhook signatures
• Added retry handling with exponential backoff and jitter
• Added an in-memory dead-letter store for exhausted webhook deliveries
• Added a subscriber registry loaded from environment configuration
• Documented outbound event schema, signing contract, retry behavior, and configuration
• Added tests for signing/verification symmetry, retries, dead-letter handling, registry loading, and no hard-coded secrets

Testing

• pnpm --filter @guildpass/webhook-utils build
• pnpm --filter @guildpass/webhook-utils typecheck
• pnpm --filter @guildpass/webhook-utils test
• pnpm --filter @guildpass/webhook-utils lint

Notes

pnpm --filter @guildpass/webhook-utils lint passes with 0 errors and 1 existing warning in packages/webhook-utils/examples/express.ts.