Add infrastructure layer with persistence and services

- EF Core DbContext and entity configurations
- Design-time factory for migrations
- Initial and soft-delete migrations
- Service implementations: Account, AccountYear, Entry,
  BalanceQuery, FileSave, PdfStatement
- Dependency injection registration

src/Duempelkas.Infrastructure/Services/BalanceQueryService.cs

@@ -0,0 +1,39 @@
+using Duempelkas.App.Services;
+using Duempelkas.App.Services.Models;
+using Duempelkas.Domain.Enums;
+using Duempelkas.Infrastructure.Persistence;
+using Microsoft.EntityFrameworkCore;
+
+namespace Duempelkas.Infrastructure.Services;
+
+public class BalanceQueryService : IBalanceQueryService
+{
+    private readonly FinanceDbContext _db;
+
+    public BalanceQueryService(FinanceDbContext db) => _db = db;
+
+    public async Task<AccountBalanceDto> GetAccountBalanceAsync(int accountId)
+    {
+        var account = await _db.Accounts
+            .Include(a => a.Entries)
+            .FirstOrDefaultAsync(a => a.Id == accountId)
+            ?? throw new InvalidOperationException($"Konto {accountId} nicht gefunden.");
+
+        var currentYear = DateTime.Now.Year;
+
+        var activeEntries = account.Entries.Where(e => !e.IsDeleted).ToList();
+        var totalIncome = activeEntries.Where(e => e.Type == EntryType.Income).Sum(e => e.Amount);
+        var totalExpense = activeEntries.Where(e => e.Type == EntryType.Expense).Sum(e => e.Amount);
+        var currentYearIncome = activeEntries.Where(e => e.Type == EntryType.Income && e.Date.Year == currentYear).Sum(e => e.Amount);
+        var currentYearExpense = activeEntries.Where(e => e.Type == EntryType.Expense && e.Date.Year == currentYear).Sum(e => e.Amount);
+        var totalBalance = account.CarryoverBalance + totalIncome - totalExpense;
+
+        return new AccountBalanceDto(
+            account.CarryoverBalance,
+            totalIncome,
+            totalExpense,
+            currentYearIncome,
+            currentYearExpense,
+            totalBalance);
+    }
+}