Implement real PayPal flow in PaymentService

[Calebux]

Summary

processPayPalPayment currently returns a mocked success payload.

Evidence

• client/lib/payment-service.ts:95
• client/lib/payment-service.ts:100

Tasks

• Integrate real PayPal order/capture flow.
• Persist provider-specific IDs and statuses.
• Handle declined/failed states explicitly.
• Add environment validation for PayPal credentials.

Acceptance Criteria

• PayPal payments produce real transaction IDs.
• Failed captures are surfaced to UI/API callers.
• Integration tests cover success/failure scenarios.