Correction to the earlier assumption (based on an ad-hoc test during
brainstorming) that sale_status=4 meant "paid". The actual mapping is
1=giao dịch thành công, 4=giao dịch thất bại - confirmed directly by the
user with real curl examples for both outcomes.

SobanleClient.changeSaleStatus now takes the status as an explicit
parameter instead of hardcoding 4. epayIPN now calls it for BOTH payment
outcomes (previously only on success), sending 1 or 4 to match the real
result - so a failed/refunded transaction's Sổ Bán Lẻ order also gets
marked accordingly instead of being silently left at "chờ xử lý" forever.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KPzWwuTEeX2vXGXvyGn4q