Add webhook support for Paddle platform

[Prateek32177]

We want to add webhook support for Paddle in tern. Paddle sends events for payments, subscriptions, customers, and more.

Tasks

• Webhook Handler:
  • Implement a handler in /src/platforms/paddlehq.ts.
  • Normalize payloads into tern’s event structure (id, type, timestamp, payload).
• Platform Config:
  • Add platformConfigs.paddlehq with supported event types (e.g., payment.succeeded, subscription.created, customer.updated).
• Test Cases:
  • Add fixtures for at least 3 event types (payment succeeded, subscription created, customer updated).
  • Write Jest tests in /tests/paddlehq.test.ts to validate parsing.
• Docs:
  • Add setup instructions in /docs/platforms.md.
  • Example: how to configure a webhook in PaddleHQ dashboard.
• Acceptance Criteria:
  • PaddleHQ handler implemented
  • Tests cover 3+ event types
  • Docs updated
• Final Integration Testing:
  • Trigger a webhook event from the PaddleHQ dashboard (use the “Send Test Event” option if available).
  • Verify the signature using the platform’s signing secret/HMAC.
  • The event should pass verification if the signature is correct.
  • The event should fail verification if the signature is invalid.
  • Add a test case in Jest covering both valid and invalid signature scenarios.

Contributor directive:

• Mention this issue and its number in your PR to get it linked with the issue.

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Helpful Links: PaddleHQ Webhooks Docs