feat: add tests for import polling, login page, money utilities, and query keys

- Implemented tests for import polling functionality, ensuring correct behavior for active and terminal import statuses.
- Created tests for the login page, validating user interactions and error handling.
- Added unit tests for money utility functions, covering parsing, formatting, and summation of monetary values.
- Developed tests for query keys, ensuring proper scoping and invalidation roots.

chore: setup testing environment and configuration

- Added setup file for testing with Vitest and Testing Library.
- Configured TypeScript and Vite for the frontend project.
- Created Docker and Nginx configurations for local development and production environments.
- Established a PostgreSQL initialization script for setting up application roles.

docs: add README and environment configuration for infra

- Included README for the infra directory, detailing local development setup and service descriptions.
- Created an example environment configuration file for local development with placeholders for sensitive information.

backend/tests/test_ledger_properties.py

@@ -0,0 +1,155 @@
+from datetime import date
+
+import pytest
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+from sqlalchemy.orm import Session
+
+from app.errors import AppError
+from app.identity.models import User
+from app.ledger.models import Posting
+from app.ledger.service import (
+    account_balance,
+    create_account,
+    create_expense,
+    create_income,
+    create_refund,
+    create_transfer,
+    get_account,
+)
+from app.spaces.service import create_space
+
+
+def seed(db: Session):
+    user = User(email="owner@example.test", password_hash="unused-in-domain-tests")
+    db.add(user)
+    db.flush()
+    space = create_space(db, user=user, name="Personal")
+    return user, space
+
+
+@given(amount=st.integers(min_value=1, max_value=10**12))
+@settings(suppress_health_check=[HealthCheck.function_scoped_fixture])
+def test_transfer_postings_conserve_money(db: Session, amount: int) -> None:
+    db.rollback()
+    user, space = seed(db)
+    source = create_account(
+        db,
+        space_id=space.id,
+        user=user,
+        name="Debit",
+        kind="debit",
+        opened_on=date(2026, 1, 1),
+        opening_amount_kopecks=amount,
+    )
+    target = create_account(
+        db,
+        space_id=space.id,
+        user=user,
+        name="Cash",
+        kind="cash",
+        opened_on=date(2026, 1, 1),
+    )
+    operation = create_transfer(
+        db,
+        space_id=space.id,
+        user=user,
+        source_account_id=source.id,
+        target_account_id=target.id,
+        amount_kopecks=amount,
+        occurred_on=date(2026, 1, 2),
+    )
+    db.flush()
+    postings = db.query(Posting).filter_by(operation_id=operation.id).all()
+    assert len(postings) == 2
+    assert sum(item.amount_kopecks for item in postings) == 0
+    assert account_balance(db, source) == 0
+    assert account_balance(db, target) == amount
+    db.rollback()
+
+
+def test_credit_expense_is_negative_balance_and_refund_is_capped(db: Session) -> None:
+    user, space = seed(db)
+    card = create_account(
+        db,
+        space_id=space.id,
+        user=user,
+        name="Credit",
+        kind="credit",
+        opened_on=date(2026, 2, 1),
+    )
+    expense = create_expense(
+        db,
+        space_id=space.id,
+        user=user,
+        account_id=card.id,
+        amount_kopecks=12_345,
+        occurred_on=date(2026, 2, 2),
+    )
+    assert account_balance(db, card) == -12_345
+    create_refund(
+        db,
+        space_id=space.id,
+        user=user,
+        parent_operation_id=expense.id,
+        amount_kopecks=2_345,
+        occurred_on=date(2026, 2, 3),
+    )
+    assert account_balance(db, card) == -10_000
+    with pytest.raises(AppError, match="Refund exceeds"):
+        create_refund(
+            db,
+            space_id=space.id,
+            user=user,
+            parent_operation_id=expense.id,
+            amount_kopecks=10_001,
+            occurred_on=date(2026, 2, 4),
+        )
+
+
+def test_cross_space_lookup_fails_closed(db: Session) -> None:
+    user, first = seed(db)
+    other = create_space(db, user=user, name="Other")
+    account = create_account(
+        db,
+        space_id=first.id,
+        user=user,
+        name="Cash",
+        kind="cash",
+        opened_on=date(2026, 1, 1),
+    )
+    with pytest.raises(AppError) as exc:
+        get_account(db, space_id=other.id, account_id=account.id)
+    assert exc.value.code == "LEDGER_ACCOUNT_NOT_FOUND"
+
+
+def test_idempotency_key_prevents_duplicate_income(db: Session) -> None:
+    user, space = seed(db)
+    account = create_account(
+        db,
+        space_id=space.id,
+        user=user,
+        name="Debit",
+        kind="debit",
+        opened_on=date(2026, 1, 1),
+    )
+    first = create_income(
+        db,
+        space_id=space.id,
+        user=user,
+        account_id=account.id,
+        amount_kopecks=50_000,
+        occurred_on=date(2026, 1, 5),
+        request_key="retry-1",
+    )
+    second = create_income(
+        db,
+        space_id=space.id,
+        user=user,
+        account_id=account.id,
+        amount_kopecks=50_000,
+        occurred_on=date(2026, 1, 5),
+        request_key="retry-1",
+    )
+    assert first.id == second.id
+    assert account_balance(db, account) == 50_000