fix: make coupon consumption atomic during submit

[my-dev-jour]

Summary

• lock the coupon row and revalidate it before consuming usage during invoice submit
• consume coupon usage inside the submit transaction instead of committing from a helper
• add regression tests for one-use usage counting and atomic consume behavior

Testing

• python3 -m py_compile pos_next/pos_next/doctype/pos_coupon/pos_coupon.py pos_next/api/invoices.py pos_next/pos_next/doctype/pos_coupon/test_pos_coupon.py
• local verification was limited to py_compile in this workspace

Closes #198

[engahmed1190]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

🤖 Generated with Claude Code

[engahmed1190]

Fix Conflicts

[my-dev-jour]

Conflicts resolved. I rebased the branch onto current develop, kept the already-merged one-use doctype logic from #205, and retained the atomic coupon-consumption changes plus regression coverage.