Add Zod validation for webhooks API

[Eldren-cmd]

Closes #952

Summary

• Adds strict Zod schemas for /api/webhooks registration, params, retry-policy updates, and signed delivery payloads.
• Adds src/routes/webhooks.ts as the route-facing wrapper for the webhook router.
• Applies boundary validation to webhook management routes with standardized structured 400 envelopes.
• Adds structured webhook registration/deletion logging and correlation IDs in webhook audit metadata.
• Documents validation behavior and retry-policy request shape in docs/webhooks.md.
• Fixes the error handler envelope helper reference so validation errors render correctly.

Testing

• jest src/validators/webhooks.test.ts src/routes/webhooks.test.ts --runInBand --forceExit --silent
• Focused tests: 16 passed
• Coverage: src/validators/webhooks.ts 100%, src/routes/webhooks.ts 100%
• Touched-file lint passed
• Filtered typecheck showed no errors in touched files

[drips-wave]

@Eldren-cmd Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[greatest0fallt1me]

Great work, this looks good. Thanks for the contribution!