Type the webhook body parameters with proper TypeScript interfaces or Zod schemas

[coderabbitai]

Context

During review of PR #154, it was identified that webhook handler functions use any type for their body parameters, which bypasses TypeScript's type checking.

Request

Refactor webhook body handling to use proper TypeScript types or Zod schemas with type guards for validation. This should cover all webhook handler functions in the codebase, particularly in src/app/api/webhook/route.ts.

References

• PR: feat: Ecommerce client management #154
• Comment: feat: Ecommerce client management #154 (comment)
• Requested by: @bassgeta