Extracts resolveChargeSource (new-card vs saved-card vs one-off nonce, with Square customer provisioning) and shared advisory-lock + post-charge recheck helpers into charge_helpers.go. Adds errors.go with chargeFailureStatus: transport/5xx/context and 429/408/425 map to 503 (retryable), structured 4xx declines map to 402, used across all four charge paths. Also fixes the no-client-key refund fallback to append a crypto/rand suffix (distinct same-amount partial refunds no longer collide) and adds refundResumeKey so legacy NULL idempotency_key rows resume with a derived key instead of an empty one.
