Fix: add missing created_by when creating trainer assignments

The assignments table has a NOT NULL constraint on created_by.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Livewire/Groups/GroupShow.php

@@ -66,6 +66,7 @@ public function assignTrainer(): void
             'status' => 'active',
             'is_primary' => false,
             'start_date' => now()->toDateString(),
+            'created_by' => auth()->id(),
         ]);
 
         $this->assignTrainerId = null;

app/Livewire/HR/TrainerShow.php

@@ -68,6 +68,7 @@ public function assignGroup(): void
             'status' => 'active',
             'is_primary' => false,
             'start_date' => now()->toDateString(),
+            'created_by' => auth()->id(),
         ]);
 
         $this->assignGroupId = null;