Build webhook payload schema documentation viewer

[Kingsman-99]

Description

WebhookConfig.tsx/src/app/dev/webhook-tester/page.tsx let users configure and test webhooks but there's no in-app reference for the actual payload schema per event type. This issue adds a schema documentation viewer.

Technical Context

New src/app/dev/webhook-docs/page.tsx and a static src/lib/webhookSchemas.ts defining the payload shape per event type (payment_received, invoice_released, invoice_refunded, etc.) with field descriptions and example JSON.

Acceptance Criteria

• Lists every webhook event type with its full payload schema (field name, type, description)
• Each schema entry includes a realistic example JSON payload, copyable to clipboard
• Schema definitions are the single source of truth also used to validate sample payloads in the webhook tester (issue Build invoice template system #11), avoiding drift between docs and tester
• Searchable/filterable by event type name
• Unit test: schema definitions are valid JSON-serializable structures for every event type

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[1sraeliteX]

Hello, I would like to work on this issue