add assignment completion api

backend/database/migrations/2026_08_10_000001_create_schedule_assignments_table.php

@@ -20,10 +20,12 @@ return new class extends Migration
                 $table->json('element_path');
                 $table->date('scheduled_date');
                 $table->unsignedInteger('position');
+                $table->timestamp('completed_at')->nullable();
                 $table->unique(['schedule_id', 'position']);
                 $table->index([
                     'schedule_id',
                     'scheduled_date',
+                    'completed_at',
                     'position',
                 ]);
             },