feat: invoke vault contract for deposit and withdraw requests

[Chucks1093]

Closes #22

Changes

• added user-signed deposit(...) and withdraw(...) contract wrappers in src/stellar/contract.ts
• reused the existing Soroban transaction build, prepare, submit, and confirmation flow for write calls
• updated /api/deposit to generate the on-chain transaction server-side and persist the returned txHash
• updated /api/withdraw to submit the vault withdrawal on-chain before creating the Prisma transaction record
• added unit coverage for the new contract wrappers and integration coverage for deposit/withdraw route behavior

Testing

• npm run lint
• npm test -- --runInBand

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