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_1.md

@@ -0,0 +1,94 @@
+---
+task_id: task_1
+story_id: story_0
+sprint_id: sprint_4
+status: not_started
+type: backend
+assignee: backend
+created_date: 2025-11-05
+completion_date: null
+---
+
+# Task 1: Add AcceptanceCriteria and Tags Fields to Story Entity
+
+## What to do
+
+Update the Story domain entity to include AcceptanceCriteria and Tags fields. These fields will be stored as JSON arrays in the database and exposed through the API.
+
+**AcceptanceCriteria**: A list of testable criteria that define when a Story is complete.
+**Tags**: A list of labels/categories for organizing and filtering Stories.
+
+## 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 properties:
+```csharp
+public List<string> AcceptanceCriteria { get; private set; } = new();
+public List<string> Tags { get; private set; } = new();
+```
+
+Add methods:
+```csharp
+public void UpdateAcceptanceCriteria(List<string> criteria)
+{
+    if (criteria != null && criteria.Count > 20)
+        throw new DomainException("Cannot have more than 20 acceptance criteria");
+
+    if (criteria != null && criteria.Any(c => c.Length > 500))
+        throw new DomainException("Each acceptance criterion cannot exceed 500 characters");
+
+    AcceptanceCriteria = criteria ?? new List<string>();
+    UpdatedAt = DateTime.UtcNow;
+}
+
+public void UpdateTags(List<string> tags)
+{
+    if (tags != null && tags.Count > 10)
+        throw new DomainException("Cannot have more than 10 tags");
+
+    if (tags != null && tags.Any(t => t.Length > 50))
+        throw new DomainException("Each tag cannot exceed 50 characters");
+
+    if (tags != null && tags.Any(t => !Regex.IsMatch(t, @"^[a-zA-Z0-9_-]+$")))
+        throw new DomainException("Tags can only contain alphanumeric characters, hyphens, and underscores");
+
+    Tags = tags ?? new List<string>();
+    UpdatedAt = DateTime.UtcNow;
+}
+```
+
+### StoryConfiguration.cs Changes
+
+Add JSON column configuration:
+```csharp
+builder.Property(s => s.AcceptanceCriteria)
+    .HasColumnType("nvarchar(max)")
+    .HasConversion(
+        v => JsonSerializer.Serialize(v, (JsonSerializerOptions)null),
+        v => JsonSerializer.Deserialize<List<string>>(v, (JsonSerializerOptions)null) ?? new List<string>())
+    .IsRequired(false);
+
+builder.Property(s => s.Tags)
+    .HasColumnType("nvarchar(max)")
+    .HasConversion(
+        v => JsonSerializer.Serialize(v, (JsonSerializerOptions)null),
+        v => JsonSerializer.Deserialize<List<string>>(v, (JsonSerializerOptions)null) ?? new List<string>())
+    .IsRequired(false);
+```
+
+## Acceptance
+
+- [ ] Story entity has AcceptanceCriteria property
+- [ ] Story entity has Tags property
+- [ ] UpdateAcceptanceCriteria method validates input
+- [ ] UpdateTags method validates input
+- [ ] EF Core configuration serializes to JSON
+- [ ] Code compiles successfully
+- [ ] Domain validation tests written
+- [ ] Tests passing