feat(transactions): enforce KYC tier transaction limits

[No-bodyq]

Description

• add UserKycTier model and persisted kycTier on users
• • add DB-backed transaction_limits configuration per KYC tier
• • seed default tier limits in migration
• • add TransactionLimitService with DB SUM aggregation and USD normalization
• • enforce limit checks before deposit, withdrawal, and swap creation
• • return 422 with DAILY_LIMIT_EXCEEDED / MONTHLY_LIMIT_EXCEEDED / SINGLE_TX_LIMIT_EXCEEDED and remainingAllowance
• • add admin CRUD endpoints for tier limits:
• • GET /admin/transaction-limits
• • POST /admin/transaction-limits
• • PATCH /admin/transaction-limits/:tier
• • add user endpoint GET /users/me/transaction-limits
• • set user kycTier during KYC approval based on submitted documents

Related Issue

Closes #308

Type of Change

• New feature
• - [x] Test

Testing Steps

• npm test -- --runInBand
• • result: 35 suites passed, 202 tests passed

Checklist

• code follows project standards
• - [x] self-review completed
• - [x] tests pass locally

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