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.Infrastructure/Data/AppDbContext.cs

@@ -23,8 +23,7 @@ public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(op
             b.HasMany(x => x.Outgos).WithOne(o => o.Budget).HasForeignKey(o => o.BudgetId).OnDelete(DeleteBehavior.Cascade);
             b.HasMany(x => x.Shares).WithOne(s => s.Budget).HasForeignKey(s => s.BudgetId).OnDelete(DeleteBehavior.Cascade);
             b.HasQueryFilter(x => !x.IsDeleted);
-            b.Property(e => e.RowVersion)
-                .HasColumnName("xmin")
+            b.Property<uint>("xmin")
                 .HasColumnType("xid")
                 .ValueGeneratedOnAddOrUpdate()
                 .IsConcurrencyToken();