feat: add StarPOS and EPAY compatibility

[zengliny]

Summary

• add StarPOS QR payment channel with RSA signing, order query, notifications, and SQL registration
• add EPAY V1/V2 compatibility endpoints for mapi, submit, notify, and return
• lock callback idempotency, protocol-version binding, signing contracts, and synthetic fixtures

Verification

• EPAY compatibility target suite: 50/50
• full payment reactor: 143/143
• reactor compile: BUILD SUCCESS
• git diff --check: passed

Notes

• production configuration and real merchant parameters are unchanged
• real MySQL migration lifecycle verification remains a separate deployment-environment task