Add unit & integration tests for payment processors and webhooks

[VetheonDevelops]

Files: lib/payments/*, lib/webhooks/*, test/

Add comprehensive unit and integration tests for payment processors (Stripe, PayPal) and webhook handlers, including error flows and edge cases.

Suggested steps:

• Add unit tests for processors with mocked provider APIs.
• Add integration tests for webhook handlers including signature verification failures and retries.
• Add CI automation for tests.

Priority: P0

[VetheonDevelops]

Partially fixed/implemented by c1b34d0