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.Api/Controllers/BudgetsController.cs

@@ -75,7 +75,14 @@ public class BudgetsController(AppDbContext db, BudgetAuthorizationService authz
         if (b is null) return NotFound();
         b.Name = req.Name;
         b.UpdatedAt = DateTimeOffset.UtcNow;
-        await db.SaveChangesAsync();
+        try
+        {
+            await db.SaveChangesAsync();
+        }
+        catch (DbUpdateConcurrencyException)
+        {
+            return Conflict(new { error = "The budget was modified by another user. Please refresh and retry." });
+        }
         return Ok(new BudgetDto(b.Id, b.Name, b.EffectiveTaxRate, b.CreatedAt, b.UpdatedAt));
     }
 
@@ -87,7 +94,8 @@ public class BudgetsController(AppDbContext db, BudgetAuthorizationService authz
         if (!await authz.IsOwnerAsync(id, userId)) return Forbid();
         var b = await db.Budgets.FindAsync(id);
         if (b is null) return NotFound();
-        db.Budgets.Remove(b);
+        b.IsDeleted = true;
+        b.DeletedAt = DateTimeOffset.UtcNow;
         await db.SaveChangesAsync();
         return NoContent();
     }

src/Budget.Api/Controllers/IncomesController.cs

@@ -86,7 +86,8 @@ public class IncomesController(AppDbContext db, BudgetAuthorizationService authz
         if (!await authz.CanWriteAsync(budgetId, userId)) return Forbid();
         var income = await db.Incomes.FirstOrDefaultAsync(i => i.Id == incomeId && i.BudgetId == budgetId);
         if (income is null) return NotFound();
-        db.Incomes.Remove(income);
+        income.IsDeleted = true;
+        income.DeletedAt = DateTimeOffset.UtcNow;
         await db.SaveChangesAsync();
         return NoContent();
     }

src/Budget.Api/Controllers/OutgosController.cs

@@ -104,7 +104,8 @@ public class OutgosController(AppDbContext db, BudgetAuthorizationService authz)
         if (!await authz.CanWriteAsync(budgetId, userId)) return Forbid();
         var outgo = await db.Outgos.FirstOrDefaultAsync(o => o.Id == outgoId && o.BudgetId == budgetId);
         if (outgo is null) return NotFound();
-        db.Outgos.Remove(outgo);
+        outgo.IsDeleted = true;
+        outgo.DeletedAt = DateTimeOffset.UtcNow;
         await db.SaveChangesAsync();
         return NoContent();
     }

src/Budget.Api/Controllers/SharesController.cs

@@ -83,7 +83,8 @@ public class SharesController(AppDbContext db, BudgetAuthorizationService authz)
         if (!await authz.IsOwnerAsync(budgetId, userId)) return Forbid();
         var share = await db.BudgetShares.FirstOrDefaultAsync(s => s.Id == shareId && s.BudgetId == budgetId);
         if (share is null) return NotFound();
-        db.BudgetShares.Remove(share);
+        share.IsDeleted = true;
+        share.DeletedAt = DateTimeOffset.UtcNow;
         await db.SaveChangesAsync();
         return NoContent();
     }

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; } = [];

src/Budget.Core/Models/BudgetShare.cs

@@ -1,6 +1,6 @@
 namespace Budget.Core.Models;
 
-public class BudgetShare
+public class BudgetShare : ISoftDeletable
 {
     public Guid Id { get; set; }
     public Guid BudgetId { get; set; }
@@ -9,6 +9,8 @@ public class BudgetShare
     public SharePermission Permission { get; set; }
     public bool IsPending { get; set; }
     public DateTimeOffset CreatedAt { get; set; }
+    public bool IsDeleted { get; set; }
+    public DateTimeOffset? DeletedAt { get; set; }
 
     public Budget Budget { get; set; } = null!;
 }

src/Budget.Core/Models/ISoftDeletable.cs

@@ -0,0 +1,7 @@
+namespace Budget.Core.Models;
+
+public interface ISoftDeletable
+{
+    bool IsDeleted { get; set; }
+    DateTimeOffset? DeletedAt { get; set; }
+}

src/Budget.Core/Models/Income.cs

@@ -1,6 +1,6 @@
 namespace Budget.Core.Models;
 
-public class Income
+public class Income : ISoftDeletable
 {
     public Guid Id { get; set; }
     public Guid BudgetId { get; set; }
@@ -8,6 +8,8 @@ public class Income
     public Frequency Frequency { get; set; }
     public decimal Amount { get; set; }
     public int SortOrder { get; set; }
+    public bool IsDeleted { get; set; }
+    public DateTimeOffset? DeletedAt { get; set; }
 
     public Budget Budget { get; set; } = null!;
 }

src/Budget.Core/Models/Outgo.cs

@@ -1,6 +1,6 @@
 namespace Budget.Core.Models;
 
-public class Outgo
+public class Outgo : ISoftDeletable
 {
     public Guid Id { get; set; }
     public Guid BudgetId { get; set; }
@@ -12,6 +12,8 @@ public class Outgo
     public string? PaymentSource { get; set; }
     public string? Notes { get; set; }
     public int SortOrder { get; set; }
+    public bool IsDeleted { get; set; }
+    public DateTimeOffset? DeletedAt { get; set; }
 
     public Budget Budget { get; set; } = null!;
 }

src/Budget.Infrastructure/Data/AppDbContext.cs

@@ -22,6 +22,12 @@ public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(op
             b.HasMany(x => x.Incomes).WithOne(i => i.Budget).HasForeignKey(i => i.BudgetId).OnDelete(DeleteBehavior.Cascade);
             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")
+                .HasColumnType("xid")
+                .ValueGeneratedOnAddOrUpdate()
+                .IsConcurrencyToken();
         });
 
         modelBuilder.Entity<Income>(b =>
@@ -29,6 +35,7 @@ public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(op
             b.HasKey(x => x.Id);
             b.Property(x => x.Name).IsRequired().HasMaxLength(200);
             b.Property(x => x.Amount).HasPrecision(18, 2);
+            b.HasQueryFilter(x => !x.IsDeleted);
         });
 
         modelBuilder.Entity<Outgo>(b =>
@@ -39,6 +46,7 @@ public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(op
             b.Property(x => x.PaymentSource).HasMaxLength(100);
             b.Property(x => x.Notes).HasMaxLength(1000);
             b.Property(x => x.Amount).HasPrecision(18, 2);
+            b.HasQueryFilter(x => !x.IsDeleted);
         });
 
         modelBuilder.Entity<KnownUser>(b =>
@@ -55,6 +63,7 @@ public class AppDbContext(DbContextOptions<AppDbContext> options) : DbContext(op
             b.Property(x => x.SharedWithUserId).HasMaxLength(200);
             b.Property(x => x.SharedWithEmail).IsRequired().HasMaxLength(200);
             b.HasIndex(x => new { x.BudgetId, x.SharedWithEmail }).IsUnique();
+            b.HasQueryFilter(x => !x.IsDeleted);
         });
     }
 }