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Consolidates GRO-605, GRO-606, GRO-608 into a single clean PR: - GRO-605: Stripe SDK integration + payment service - GRO-606: Payment API endpoints (pay invoice, payment methods, refunds) - GRO-608: Stripe webhook handler Migration consolidation: - Single 0026_stripe_payment.sql migration adds stripeCustomerId to clients and stripe_payment_intent_id, stripe_refund_id, payment_failure_reason to invoices - Removed duplicate 0027_stripe_identifiers.sql Co-Authored-By: Paperclip <noreply@paperclip.ing>