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_4.md

@@ -0,0 +1,114 @@
+---
+task_id: task_4
+story_id: story_0
+sprint_id: sprint_4
+status: not_started
+type: backend
+assignee: backend
+created_date: 2025-11-05
+completion_date: null
+---
+
+# Task 4: Create and Apply Database Migration
+
+## What to do
+
+Create an EF Core migration to add the new fields (AcceptanceCriteria, Tags, StoryPoints on Story; Order on Task) to the database.
+
+## Files to create
+
+- `colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Infrastructure/Migrations/YYYYMMDDHHMMSS_AddAdvancedStoryTaskFields.cs`
+
+## Implementation
+
+### Migration Creation
+
+Run EF Core migration command:
+```bash
+cd colaflow-api/src/Modules/ProjectManagement/ColaFlow.Modules.ProjectManagement.Infrastructure
+dotnet ef migrations add AddAdvancedStoryTaskFields --context ProjectManagementDbContext
+```
+
+### Expected Migration Content
+
+**Up Migration**:
+```csharp
+migrationBuilder.AddColumn<string>(
+    name: "AcceptanceCriteria",
+    table: "Stories",
+    type: "nvarchar(max)",
+    nullable: true);
+
+migrationBuilder.AddColumn<string>(
+    name: "Tags",
+    table: "Stories",
+    type: "nvarchar(max)",
+    nullable: true);
+
+migrationBuilder.AddColumn<int>(
+    name: "StoryPoints",
+    table: "Stories",
+    type: "int",
+    nullable: true);
+
+migrationBuilder.AddColumn<int>(
+    name: "Order",
+    table: "Tasks",
+    type: "int",
+    nullable: false,
+    defaultValue: 0);
+
+migrationBuilder.CreateIndex(
+    name: "IX_Tasks_StoryId_Order",
+    table: "Tasks",
+    columns: new[] { "StoryId", "Order" });
+```
+
+**Down Migration**:
+```csharp
+migrationBuilder.DropIndex(
+    name: "IX_Tasks_StoryId_Order",
+    table: "Tasks");
+
+migrationBuilder.DropColumn(name: "AcceptanceCriteria", table: "Stories");
+migrationBuilder.DropColumn(name: "Tags", table: "Stories");
+migrationBuilder.DropColumn(name: "StoryPoints", table: "Stories");
+migrationBuilder.DropColumn(name: "Order", table: "Tasks");
+```
+
+### Data Backfill (Optional)
+
+Add SQL to set Task Order based on CreatedAt:
+```csharp
+migrationBuilder.Sql(@"
+    WITH OrderedTasks AS (
+        SELECT Id, ROW_NUMBER() OVER (PARTITION BY StoryId ORDER BY CreatedAt) - 1 AS NewOrder
+        FROM Tasks
+    )
+    UPDATE t
+    SET t.[Order] = ot.NewOrder
+    FROM Tasks t
+    INNER JOIN OrderedTasks ot ON t.Id = ot.Id;
+");
+```
+
+### Apply Migration
+
+Run migration locally:
+```bash
+cd colaflow-api/src/ColaFlow.API
+dotnet ef database update --context ProjectManagementDbContext
+```
+
+## Acceptance
+
+- [ ] Migration file created
+- [ ] Migration adds AcceptanceCriteria column (nvarchar(max), nullable)
+- [ ] Migration adds Tags column (nvarchar(max), nullable)
+- [ ] Migration adds StoryPoints column (int, nullable)
+- [ ] Migration adds Order column (int, not null, default 0)
+- [ ] Migration creates composite index (StoryId, Order)
+- [ ] Migration applied successfully to local database
+- [ ] Schema verified in database
+- [ ] Rollback migration tested
+- [ ] Migration script committed