feat(api): invoke vault contract on deposit/withdraw

[edehvictor]

Closes #32

Changes

• add typed depositForUser and withdrawForUser helpers in src/stellar/contract.ts
• use the existing Soroban build/submit/confirm flow for vault write calls
• make the signing strategy explicit: custodial backend signing via src/stellar/wallet.ts
• update POST /api/deposit and POST /api/withdraw to submit on-chain transactions
• persist the real on-chain txHash and confirmed status in Prisma transactions
• return a stable API response including top-level txHash and status
• document the custodial signing model in src/stellar/README.md
• mock Stellar contract calls in API tests and expand coverage for deposit/withdraw behavior

Signing Strategy

This implementation uses a custodial model. The backend decrypts the user’s encrypted Stellar secret from src/stellar/wallet.ts, signs the Soroban transaction server-side, and submits it through the configured RPC flow. Secrets are not returned or logged.

Testing

• node .\node_modules\typescript\bin\tsc --noEmit
• node .\node_modules\jest\bin\jest.js --runInBand

Notes

• I could not fetch upstream from Neurowealth/Backend in this environment, so this branch is based on the current local main rather than a verified upstream-rebased main.

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