feat: wallet clone - atomic transfers, append-only ledger, integer paise (bounty #6)

[Kasuki354]

Summary

Implements bounty #6: Wallet clone (atomic transfers + ledger + integer paise + idempotency)

Invariant

50 concurrent full-balance debits of one account -> exactly one succeeds, balance never goes negative, zero 5xx. Same idempotency key + different body -> 409.

Implementation

Backend:

• Wallet model: id, ownerId (unique), balancePaise (BigInt), cloneSlug
• LedgerEntry model: append-only, fromWalletId, toWalletId, amountPaise (BigInt), idempotencyKey (unique), fingerprint
• POST /clones/:slug/wallets: create wallet with initial balance
• GET /wallets/:id: check balance
• POST /clones/:slug/transfers: atomic transfer via prisma. (debit + credit + ledger entry in one tx)
  • Insufficient balance -> 409 (checked inside transaction)
  • Self-transfer -> 400
  • Idempotency via unique on idempotencyKey: same key + same body -> 200, same key + different body -> 409
  • Wallet not found -> 404
• GET /clones/:slug/ledger: last 100 ledger entries (append-only audit trail)
• All amounts as BigInt paise (1 rupee = 100 paise, no floating point)
• Metrics updated with wallet + transfer counts

Frontend:

• /clones/wallet page with create wallet, transfer, and check balance forms

Tests:

• Happy path: create + transfer + balance check
• Idempotent retry: same key + same body -> 200
• Concurrency: 50 concurrent full-balance debits -> 1 success + 49 rejected + zero 5xx
• Error: key conflict (409), insufficient balance (409), 404, negative/zero amount (400), self-transfer (400)

Resolves #6