[#452] Backend: Replace mock referral yield calculation with real on-chain or ledger-backed accounting

[UzyKhs]

Summary

• replaced mock referral yield multiplier logic with ledger/snapshot-backed net-yield accounting
• added share price snapshot persistence model and Prisma migration
• updated referral integration tests for deterministic deposit/withdrawal scenarios and zero/negative yield handling
• added structured referral calculation summary logs without sensitive wallet details

Notes

• output precision remains 6 decimal places and response schema is unchanged
• reward is only accrued for positive net yield scenarios

Closes Junirezz#452