fix(tasks): clear assignee relation when updating assignedTo FK

TypeORM caches the loaded relation object. When updating only the FK
column (assignedTo), the cached assignee relation would override the
new FK value during save. Clearing the relation ensures the new
assignedTo value is properly persisted.

Author: youssefx99

src/tasks/tasks.service.ts

@@ -75,6 +75,14 @@ export class TasksService {
 
   async update(id: string, updateTaskDto: UpdateTaskDto): Promise<Task> {
     const task = await this.findOne(id);
+    
+    // ROOT FIX: Clear the assignee relation when assignedTo changes
+    // This ensures TypeORM properly updates the FK and doesn't cache stale relation
+    if (updateTaskDto.assignedTo !== undefined) {
+      // Clear the loaded relation so TypeORM uses the new FK value
+      task.assignee = null as any;
+    }
+    
     Object.assign(task, updateTaskDto);
     const updatedTask = await this.taskRepository.save(task);