fix(death): add form_number to DeathCase model fillable array

form_number was being stripped by the model's mass-assignment guard,
so it was never saved to the database despite being in the POST data.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Modules/Death/Models/DeathCase.php

@@ -22,7 +22,7 @@ class DeathCase extends Model
         'spouse_id', 'primary_spouse_id', 'secondary_spouses_json',
         'primary_spouse_form_filled', 'child_id',
         'children_assignment_json', 'transferred_to_member_id',
-        'same_membership_number', 'fee_amount',
+        'same_membership_number', 'fee_amount', 'form_number',
         'board_fee_method', 'board_fee_value', 'board_fee_amount',
         'board_decision_reference', 'board_decision_date', 'board_notes',
         'board_approved_by', 'board_approved_at', 'payment_request_id',