Phase 5: Add EF migration for soft delete columns; use shadow property for xmin

- Migration adds IsDeleted/DeletedAt to Budgets, Incomes, Outgos, BudgetShares
- xmin concurrency token configured as shadow property (uint, xid type) — not added
  as a column in the migration since xmin is a PostgreSQL system column
- Budget.RowVersion property removed; concurrency tracked via shadow property only

src/Budget.Core/Models/Budget.cs

@@ -10,7 +10,6 @@ public class Budget : ISoftDeletable
     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; } = [];