Expert Tier System (Bronze / Silver / Gold)

[Luluameh]

Background
High-performing experts deserve better visibility and lower platform fees. A tiered system incentivises quality.
Goal
Assign experts to tiers based on completed session count and average rating. Tiers unlock fee discounts and badge NFTs.
Acceptance Criteria

• ExpertTier enum: Bronze, Silver, Gold.
• Tier recalculated and stored on each session completion.
• Gold experts pay 0 platform fee, Silver 50% discount, Bronze full fee.
• Emits TierUpgraded { expert, old_tier, new_tier } event.
  Key Files
  contracts/src/reputation.rs, contracts/src/lib.rs