feat(traders): implement fee discount tier indexing and discount lookup API (#1030)

[yodakkak]

Closes #1030

Summary

• Added GET /api/v1/traders/:wallet/discount (src/modules/traders/trader-discount.routes.ts) returning the wallet's current tier, discount bps, and 30-day rolling trading volume cached with a 5-minute TTL.
• Added indexVolumeDiscountTierChangedEvent in src/modules/traders/trader-discount.service.ts with idempotent event deduplication and automatic cache refresh.
• Included discountTier in the dynamic fee breakdown response (src/modules/keys/key-dynamic-fee.service.ts).
• Added unit tests in src/modules/traders/trader-discount.service.test.ts.

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Payout Wallet (Stellar / USDC): GD3TVG3SLNQMK7VS7772FZPNM5LTH24NR5NSWMURN2CXEJUDA4V2XONU
EVM Wallet: 0x59622Fe8f9b438eF1CD5dB4e91f1230fe6239AC9