Build referral tracking and reward distribution (#910)

[Stanley-Owoh]

Summary

• Add authenticated referral registration, earnings, and referred-wallet endpoints.
• Generate and persist a unique referral code per wallet, and enforce one referrer per referee (duplicate registration returns 409).
• Record a configured share of a referred wallet's first buy or sell as a referral reward, with an atomic first-trade claim and fee-ledger write.
• Add Prisma schema/migration, configuration docs, and focused service, route, and indexer tests.

Verification

• git diff --check passed; editor diagnostics reported no errors in the modified TypeScript files.
• Jest/build and the pre-commit hook could not run because Node.js and pnpm are unavailable in the environment.

Closes #910

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