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_2.md

@@ -0,0 +1,65 @@
+---
+task_id: task_2
+story_id: story_0
+sprint_id: sprint_4
+status: not_started
+type: backend
+assignee: backend
+created_date: 2025-11-05
+completion_date: null
+---
+
+# Task 2: Add StoryPoints Field to Story Entity
+
+## What to do
+
+Add a StoryPoints field to the Story entity for estimating Story complexity using Fibonacci numbers (1, 2, 3, 5, 8, 13, 21, etc.).
+
+## Files to modify
+
+- `colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Domain/Aggregates/ProjectAggregate/Story.cs`
+- `colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Infrastructure/Persistence/Configurations/StoryConfiguration.cs`
+
+## Implementation
+
+### Story.cs Changes
+
+Add property:
+```csharp
+public int? StoryPoints { get; private set; }
+```
+
+Add method:
+```csharp
+public void UpdateStoryPoints(int? points)
+{
+    if (points.HasValue)
+    {
+        if (points.Value < 0)
+            throw new DomainException("Story points cannot be negative");
+
+        if (points.Value > 100)
+            throw new DomainException("Story points cannot exceed 100");
+    }
+
+    StoryPoints = points;
+    UpdatedAt = DateTime.UtcNow;
+}
+```
+
+### StoryConfiguration.cs Changes
+
+Add column configuration:
+```csharp
+builder.Property(s => s.StoryPoints)
+    .IsRequired(false);
+```
+
+## Acceptance
+
+- [ ] Story entity has StoryPoints property (nullable int)
+- [ ] UpdateStoryPoints method validates range (0-100)
+- [ ] EF Core configuration includes StoryPoints column
+- [ ] Code compiles successfully
+- [ ] Validation tests written
+- [ ] Tests passing