Tier-based fee rebates & discounts

[DevMuhdishaq]

Description
get_user_tier exists and swaps use tiered fees; formalize a configurable fee-discount schedule so higher tiers pay proportionally lower fees.

Direction

• In tiers.rs/fee_progression, define a tier→discount-bps table (admin-configurable).
• Apply the discount inside the swap fee computation; expose get_effective_fee_bps(user, pair).
• Ensure discounts compose safely with the dynamic-fee suite (no negative/underflow fees).

Acceptance criteria

• Higher-tier users are charged a lower effective fee on identical swaps.
• get_effective_fee_bps matches the fee actually deducted in a swap.
• Discount cannot drive fees below zero.
• Admin can update the discount table; non-admin cannot.
• Tests cover each tier boundary.

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[demilade18-git]

can i take this up?

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #220)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@demilade18-git's PR #233 was approved and merged by @DevMuhdishaq.

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