feat: persist stellarPublicKey to User record after account creation

[Codex723]

TITLE: Persist generated stellarPublicKey to the User record

Description

This PR implements issue #5 to persist the generated stellarPublicKey to the User record after calling createTestnetAccount().

Changes

Main Implementation

• src/app/api/stellar/account/route.ts:
  • Added db import from @/lib/db
  • Added check for existing stellarPublicKey - returns 409 Conflict if user already has one
  • Added db.user.update() call to persist the stellarPublicKey to the user record after createTestnetAccount() succeeds
  • Removed the TODO(contributor) comment since the feature is now implemented

Test Suite

• src/app/api/stellar/account/route.test.ts:
  • Created comprehensive test suite with 4 test cases covering all acceptance criteria:
    • Returns 401 when user is not authenticated
    • Returns 409 when user already has a stellarPublicKey
    • Creates a Stellar account and updates user record when user has no stellarPublicKey
    • Returns 500 when createTestnetAccount fails

Infrastructure

• vitest.config.ts: Added Vitest configuration for test configuration
• src/test/setup.ts: Added test environment setup
• package.json: Added test and test:api scripts
• .github/workflows/backendci.yml: Updated to run API tests in CI

Bug Fixes

• contracts/escrow/src/lib.rs: Fixed pre-existing contract compilation errors (added .clone() to escrow_id in release() and refund() functions)
• contracts/escrow/src/test.rs: Fixed pre-existing test file errors (removed unused imports)

Acceptance Criteria

• User.stellarPublicKey is set after calling POST /api/stellar/account
• Returns 409 if user already has a stellarPublicKey
• Covered by a route test

How to Test

Run npm run test to execute the test suite.