Add Polar Webhook Support

[Prateek32177]

We want to add webhook support for Polar in tern. Polar sends events for sponsorships, payouts, and other platform activities.

Tasks

Webhook Handler

• Implement /src/platforms/polar.ts.
• Normalize payloads into tern’s event format.

Platform Config

• Add platformConfigs.polar with supported events (e.g., sponsorships/created, payouts/processed).

Test Cases

• Add JSON fixtures for 3+ events (sponsorship created, payout processed, etc.).
• Write Jest tests in /tests/polar.test.ts.

Docs

• Update /docs/platforms.md with setup steps.
• Mention secret verification/manual webhook verification. See Polar Webhook Verification Docs

Acceptance Criteria

• Polar handler implemented
• Manual or secret-based webhook verification supported
• Tests with 3+ event types
• Docs updated

Helpful Links: Polar Webhooks Docs