[5.3] Integration tests: 3 end-to-end test scenarios

[lomartins]

Phase 5.3 — Integration tests (Medium)

Write 3 #[tokio::test] tests in backend/tests/integration_test.rs using the scaffolding Claude provides.

Test 1: test_create_user_and_get_it_back

POST a user, GET by id, assert the returned email matches.

Test 2: test_create_account_requires_valid_user

POST an account for a nonexistent user_id, assert the response status is 404.

Test 3: test_transaction_updates_account_balance

1. Create a user + account
2. POST a transaction with amount: "50.00"
3. GET the account
4. Assert balance == "50.00"

Prerequisite: Phase 5.2 integration test scaffolding must be complete (Claude task).

Done when: cargo test (with Docker running) — all tests pass.