Referral Fee Auto-Distribution at Settlement

[Luluameh]

Background
Issue 52 designs a referral program. This issue implements the on-chain settlement logic: referral fees should be paid out automatically when a referred expert earns from a session.
Goal
During settle_session, if the expert has a referrer, calculate and transfer the referral commission atomically.
Acceptance Criteria

• Referral commission = expert_earnings * referral_bps / 10000.
• Deducted from platform fee, not expert earnings.
• Commission only applies for the first 50 sessions of a referred expert (configurable).
• Emits ReferralCommissionPaid { referrer, referee, amount }.
  Key Files
  contracts/src/lib.rs, contracts/src/governance.rs