feat(expense): add MonthlyStatementService with server-side recompute + tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

API/ROLAC.API/Services/IMonthlyStatementService.cs

@@ -0,0 +1,11 @@
+using ROLAC.API.DTOs.Expense;
+namespace ROLAC.API.Services;
+
+public interface IMonthlyStatementService
+{
+    Task<List<MonthlyStatementDto>> GetAllAsync(int? year);
+    Task<MonthlyStatementDto?> GetByIdAsync(int id);
+    Task<int> CreateAsync(CreateMonthlyStatementRequest r);
+    Task UpdateAsync(int id, UpdateMonthlyStatementRequest r);
+    Task FinalizeAsync(int id);
+}

API/ROLAC.API/Services/MonthlyStatementService.cs

@@ -0,0 +1,93 @@
+using System.Security.Claims;
+using Microsoft.EntityFrameworkCore;
+using ROLAC.API.Data;
+using ROLAC.API.DTOs.Expense;
+using ROLAC.API.Entities;
+
+namespace ROLAC.API.Services;
+
+public class MonthlyStatementService : IMonthlyStatementService
+{
+    private readonly AppDbContext _db;
+    private readonly IHttpContextAccessor _http;
+    public MonthlyStatementService(AppDbContext db, IHttpContextAccessor http) { _db = db; _http = http; }
+
+    private string CurrentUserId =>
+        _http.HttpContext?.User.FindFirstValue(ClaimTypes.NameIdentifier) ?? "system";
+
+    public async Task<List<MonthlyStatementDto>> GetAllAsync(int? year)
+    {
+        var query = _db.MonthlyStatements.AsNoTracking().AsQueryable();
+        if (year.HasValue) query = query.Where(s => s.Year == year.Value);
+        var list = await query.OrderByDescending(s => s.Year).ThenByDescending(s => s.Month).ToListAsync();
+        return list.Select(Map).ToList();
+    }
+
+    public async Task<MonthlyStatementDto?> GetByIdAsync(int id)
+    {
+        var s = await _db.MonthlyStatements.AsNoTracking().FirstOrDefaultAsync(x => x.Id == id);
+        return s is null ? null : Map(s);
+    }
+
+    public async Task<int> CreateAsync(CreateMonthlyStatementRequest r)
+    {
+        if (await _db.MonthlyStatements.AnyAsync(s => s.Year == r.Year && s.Month == r.Month))
+            throw new InvalidOperationException($"A statement for {r.Year}-{r.Month:D2} already exists.");
+
+        var s = new MonthlyStatement
+        {
+            Year = r.Year, Month = r.Month,
+            OpeningBalance = r.OpeningBalance, TotalOtherIncome = r.TotalOtherIncome,
+            BankStatementBalance = r.BankStatementBalance, Notes = r.Notes,
+        };
+        await RecomputeAsync(s);
+        _db.MonthlyStatements.Add(s);
+        await _db.SaveChangesAsync();
+        return s.Id;
+    }
+
+    public async Task UpdateAsync(int id, UpdateMonthlyStatementRequest r)
+    {
+        var s = await _db.MonthlyStatements.FindAsync(id)
+            ?? throw new KeyNotFoundException($"MonthlyStatement {id} not found.");
+        if (s.IsFinalized) throw new InvalidOperationException("Statement is finalized and cannot be modified.");
+        s.OpeningBalance = r.OpeningBalance; s.TotalOtherIncome = r.TotalOtherIncome;
+        s.BankStatementBalance = r.BankStatementBalance; s.Notes = r.Notes;
+        await RecomputeAsync(s);
+        await _db.SaveChangesAsync();
+    }
+
+    public async Task FinalizeAsync(int id)
+    {
+        var s = await _db.MonthlyStatements.FindAsync(id)
+            ?? throw new KeyNotFoundException($"MonthlyStatement {id} not found.");
+        s.IsFinalized = true; s.FinalizedAt = DateTimeOffset.UtcNow; s.FinalizedBy = CurrentUserId;
+        await _db.SaveChangesAsync();
+    }
+
+    private async Task RecomputeAsync(MonthlyStatement s)
+    {
+        var first = new DateOnly(s.Year, s.Month, 1);
+        var next  = first.AddMonths(1);
+
+        s.TotalGiving = await _db.Givings
+            .Where(g => g.GivingDate >= first && g.GivingDate < next)
+            .SumAsync(g => (decimal?)g.Amount) ?? 0m;
+
+        s.TotalExpenses = await _db.Expenses
+            .Where(e => e.Status == "Paid" && e.ExpenseDate >= first && e.ExpenseDate < next)
+            .SumAsync(e => (decimal?)e.Amount) ?? 0m;
+
+        s.CalculatedClosingBalance = s.OpeningBalance + s.TotalGiving + s.TotalOtherIncome - s.TotalExpenses;
+        s.Difference = s.CalculatedClosingBalance - s.BankStatementBalance;
+    }
+
+    private static MonthlyStatementDto Map(MonthlyStatement s) => new()
+    {
+        Id = s.Id, Year = s.Year, Month = s.Month,
+        OpeningBalance = s.OpeningBalance, TotalGiving = s.TotalGiving, TotalOtherIncome = s.TotalOtherIncome,
+        TotalExpenses = s.TotalExpenses, CalculatedClosingBalance = s.CalculatedClosingBalance,
+        BankStatementBalance = s.BankStatementBalance, Difference = s.Difference,
+        Notes = s.Notes, IsFinalized = s.IsFinalized,
+    };
+}