Add proper test mode / sandbox for integrations

Currently, there is no dedicated test mode or sandbox environment to fully test affiliate tracking setups (e.g. with Stripe, Polar or Paddle). The only reliable way to test end-to-end is by using a 100% discount code in live mode.

Provide a dedicated test mode or sandbox environment where users can:

• Simulate referrals, checkouts, and conversions

• Validate tracking (e.g. metadata, attribution)

• Test integrations without requiring live payment setups

Current workaround:

• Use 100% discount codes in live mode

• Or manually inspect metadata in dev logs

Notes / Use cases:

• Faster onboarding and debugging

• Safer testing without touching live environments

• Especially useful for new users setting up integrations for the first time