fix: Add JSON string enum converter for Issue Management API

- Configure AddControllers() with JsonStringEnumConverter
- Allows API to accept Issue type/priority/status as strings ("Story", "High", "Backlog")
- Frontend can now send readable enum values instead of integers
- All Issue Management CRUD operations tested and working

Test results:
- Create Issue (Story, Bug, Task) ✓
- List all issues ✓
- Filter by status (Backlog, InProgress) ✓
- Change issue status (Kanban workflow) ✓
- Update issue details ✓
- Multi-tenant isolation verified ✓

colaflow-api/src/ColaFlow.API/Program.cs

@@ -44,8 +44,12 @@ builder.Services.Configure<Microsoft.AspNetCore.ResponseCompression.GzipCompress
     options.Level = System.IO.Compression.CompressionLevel.Fastest;
 });
 
-// Add controllers
-builder.Services.AddControllers();
+// Add controllers with JSON string enum converter
+builder.Services.AddControllers()
+    .AddJsonOptions(options =>
+    {
+        options.JsonSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
+    });
 
 // Configure exception handling (IExceptionHandler - .NET 8+)
 builder.Services.AddExceptionHandler<GlobalExceptionHandler>();