remove subject persistence and keep params payload

Made-with: Cursor

app/Jobs/AssembleExamTaskJob.php

@@ -183,7 +183,6 @@ class AssembleExamTaskJob implements ShouldQueue
                 'paper_name' => $paperName,
                 'student_id' => $data['student_id'],
                 'teacher_id' => $data['teacher_id'] ?? null,
-                'subject' => $data['subject'] ?? null,
                 'params' => $requestPayloadParams,
                 'assembleType' => $finalAssembleType,
                 'difficulty_category' => $difficultyCategory,

app/Models/Paper.php

@@ -19,7 +19,6 @@ class Paper extends Model
         'paper_id',
         'student_id',
         'teacher_id',
-        'subject',
         'params',
         'paper_name',
         'paper_type',
@@ -40,7 +39,6 @@ class Paper extends Model
         'paper_id' => 'string',
         'student_id' => 'string',
         'teacher_id' => 'string',
-        'subject' => 'string',
         'params' => 'array',
         'total_questions' => 'integer',
         'total_score' => 'float',

app/Services/QuestionBankService.php

@@ -585,7 +585,6 @@ class QuestionBankService
                     'paper_id' => $paperId,
                     'student_id' => $studentId,
                     'teacher_id' => $examData['teacher_id'] ?? '',
-                    'subject' => $examData['subject'] ?? null,
                     'params' => $examData['params'] ?? null,
                     'paper_name' => $paperDisplayName,
                     'paper_type' => $assembleType, // assemble_type 唯一来源