Phase 1-4: Add soft delete, concurrency token, update EF config and controllers

- Add ISoftDeletable interface with IsDeleted/DeletedAt
- Implement on Budget, Income, Outgo, BudgetShare; add RowVersion (xmin) to Budget
- Configure EF global query filters and xmin concurrency token
- Replace Remove() with soft delete in all delete endpoints
- Wrap Budget.Update SaveChanges in DbUpdateConcurrencyException catch

src/Budget.Core/Models/Budget.cs

@@ -1,6 +1,6 @@
 namespace Budget.Core.Models;
 
-public class Budget
+public class Budget : ISoftDeletable
 {
     public Guid Id { get; set; }
     public required string Name { get; set; }
@@ -8,6 +8,9 @@ public class Budget
     public decimal EffectiveTaxRate { get; set; } = 0.25m;
     public DateTimeOffset CreatedAt { get; set; }
     public DateTimeOffset UpdatedAt { get; set; }
+    public bool IsDeleted { get; set; }
+    public DateTimeOffset? DeletedAt { get; set; }
+    public byte[] RowVersion { get; set; } = [];
 
     public List<Income> Incomes { get; set; } = [];
     public List<Outgo> Outgos { get; set; } = [];