Implement dynamic fee rate computation endpoint

[Chucks1093]

Summary

Expose an endpoint that computes the effective dynamic fee rate for a given trade, factoring in base fee, volume-tier discount, protocol fee, and creator royalty.

Scope

• GET /keys/:keyId/fee?amount=&direction=buy|sell endpoint
• Fee components: base, volume tier, protocol, royalty returned individually
• Volume tier discount computed from wallet 30-day volume
• Effective rate and total fee in key units and USD returned
• Fee config cached from contract with 5-minute TTL

Acceptance Criteria

• All four fee components computed and returned correctly
• Volume tier discount applied correctly based on wallet history
• Effective rate and totals returned in both units
• Fee config cache refreshed every 5 minutes
• Edge cases: zero amount and max amount handled without error

ETA

12 hours

[eriifeomoniyi-hue]

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[Samuel-21-eng]

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I would like to work on this one

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[whitezaddy]

@whitezaddy has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

I can resolve this issue

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[Cmitchelle7]

@Cmitchelle7 has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hi Maintainer, reviewed the issue in detail. Full stack background with solid experience relevant to this task. No blockers on my end — I'm ready to submit a PR shortly. Please assign.

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