fix: REFUND_FEE_BPS is hardcoded — make admin-configurable via instance storage

[Pvsaint]

Problem

The refund processing fee is hardcoded as a compile-time constant:

const REFUND_FEE_BPS: i128 = 100; // 1% — lib.rs line 16

This cannot be changed post-deployment without a full contract upgrade. As business requirements evolve (promotions, regulatory changes, tiered merchants), the inability to adjust fees is a significant operational limitation.

Proposed Solution

Store refund_fee_bps in instance storage, initialized during initialize_refund_manager and updatable by admin.

DataKey::RefundFeeBps  // instance storage key

Add:

• set_refund_fee_bps(env, admin, bps: i128) — admin-only setter
• get_refund_fee_bps(env) → i128 — public getter

Acceptance Criteria

• REFUND_FEE_BPS constant is replaced by a storage-backed value
• Default value of 100 bps (1%) is set during contract initialization
• set_refund_fee_bps requires admin auth and validates 0 <= bps <= 1000 (max 10%)
• get_refund_fee_bps is publicly readable
• process_refund reads from storage rather than the constant
• CHANGELOG.md notes this as a storage key change (bumps version if storage layout changes)
• Tests cover: default fee, fee update by admin, fee update rejected by non-admin, fee applied correctly

Branch & Commit Examples

branch: feat/configurable-refund-fee
commit: feat(refund-manager): make refund fee bps admin-configurable via instance storage