feat(api): add endpoints for importing files and getting account balance

app/apps/accounts/services.py

@@ -0,0 +1,33 @@
+from decimal import Decimal
+
+from django.db import models
+
+from apps.accounts.models import Account
+from apps.transactions.models import Transaction
+
+
+def get_account_balance(account: Account, paid_only: bool = True) -> Decimal:
+    """
+    Calculate account balance (income - expense).
+
+    Args:
+        account: Account instance to calculate balance for.
+        paid_only: If True, only count paid transactions (current balance).
+                   If False, count all transactions (projected balance).
+
+    Returns:
+        Decimal: The calculated balance (income - expense).
+    """
+    filters = {"account": account}
+    if paid_only:
+        filters["is_paid"] = True
+
+    income = Transaction.objects.filter(
+        type=Transaction.Type.INCOME, **filters
+    ).aggregate(total=models.Sum("amount"))["total"] or Decimal("0")
+
+    expense = Transaction.objects.filter(
+        type=Transaction.Type.EXPENSE, **filters
+    ).aggregate(total=models.Sum("amount"))["total"] or Decimal("0")
+
+    return income - expense