docs(backend): Add Sprint 4 backend API verification and optional enhancement story

Backend APIs are 100% ready for Sprint 4 frontend implementation. Created comprehensive verification report and optional enhancement story for advanced UX fields.

Changes:
- Created backend_api_verification.md (detailed API analysis)
- Created Story 0: Backend API Enhancements (optional P2)
- Created 6 tasks for Story 0 implementation
- Updated Sprint 4 to include backend verification status
- Verified Story/Task CRUD APIs are complete
- Documented missing optional fields (AcceptanceCriteria, Tags, StoryPoints, Order)
- Provided workarounds for Sprint 4 MVP

Backend Status:
- Story API: 100% complete (8 endpoints)
- Task API: 100% complete (9 endpoints)
- Security: Multi-tenant isolation verified
- Missing optional fields: Can be deferred to future sprint

Frontend can proceed with P0/P1 Stories without blockers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/plans/sprint_4_story_0_task_3.md

@@ -0,0 +1,80 @@
+---
+task_id: task_3
+story_id: story_0
+sprint_id: sprint_4
+status: not_started
+type: backend
+assignee: backend
+created_date: 2025-11-05
+completion_date: null
+---
+
+# Task 3: Add Order Field to Task Entity
+
+## What to do
+
+Add an Order field to the WorkTask entity to support manual task reordering via drag-and-drop in the frontend.
+
+## Files to modify
+
+- `colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Domain/Aggregates/ProjectAggregate/WorkTask.cs`
+- `colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Infrastructure/Persistence/Configurations/WorkTaskConfiguration.cs`
+
+## Implementation
+
+### WorkTask.cs Changes
+
+Add property:
+```csharp
+public int Order { get; private set; }
+```
+
+Update Create method to set default Order:
+```csharp
+public static WorkTask Create(TenantId tenantId, string title, string description, StoryId storyId, TaskPriority priority, UserId createdBy)
+{
+    // Existing validation...
+
+    return new WorkTask
+    {
+        // Existing fields...
+        Order = 0, // Default order
+        // ...
+    };
+}
+```
+
+Add method:
+```csharp
+public void UpdateOrder(int newOrder)
+{
+    if (newOrder < 0)
+        throw new DomainException("Task order cannot be negative");
+
+    Order = newOrder;
+    UpdatedAt = DateTime.UtcNow;
+}
+```
+
+### WorkTaskConfiguration.cs Changes
+
+Add column configuration and index:
+```csharp
+builder.Property(t => t.Order)
+    .IsRequired()
+    .HasDefaultValue(0);
+
+builder.HasIndex(t => new { t.StoryId, t.Order })
+    .HasDatabaseName("IX_Tasks_StoryId_Order");
+```
+
+## Acceptance
+
+- [ ] WorkTask entity has Order property (int, default 0)
+- [ ] Create method sets default Order to 0
+- [ ] UpdateOrder method validates non-negative
+- [ ] EF Core configuration includes Order column
+- [ ] Composite index created (StoryId, Order)
+- [ ] Code compiles successfully
+- [ ] Validation tests written
+- [ ] Tests passing