fix: persist knowledge pdf url into papers table

Upsert a papers row when knowledge explanation PDF is generated so callback consumers that query papers by paper_id can resolve all_pdf_url reliably.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/Jobs/GenerateKnowledgeExplanationPdfJob.php

@@ -3,6 +3,7 @@
 namespace App\Jobs;
 
 use App\Models\KnowledgeExplanation;
+use App\Models\Paper;
 use App\Services\ExamPdfExportService;
 use App\Services\TaskManager;
 use Illuminate\Bus\Queueable;
@@ -56,6 +57,7 @@ class GenerateKnowledgeExplanationPdfJob implements ShouldQueue
                 'pdf_url' => $pdfUrl,
                 'generated_at' => now(),
             ]);
+            $this->syncPaperRecord($record, $pdfUrl);
 
             $taskManager->markTaskCompleted($this->taskId, [
                 'paper_id' => $this->knowledgeId,
@@ -86,4 +88,40 @@ class GenerateKnowledgeExplanationPdfJob implements ShouldQueue
             ->update(['status' => 'failed']);
         app(TaskManager::class)->markTaskFailed($this->taskId, $exception->getMessage());
     }
+
+    private function syncPaperRecord(KnowledgeExplanation $record, string $pdfUrl): void
+    {
+        $paperId = (string) ($record->knowledge_id ?? '');
+        if ($paperId === '') {
+            return;
+        }
+
+        $displayCode = (string) preg_replace('/^(paper_|knowledge_)/', '', $paperId);
+        if ($displayCode === '') {
+            $displayCode = $paperId;
+        }
+
+        Paper::query()->updateOrCreate(
+            ['paper_id' => $paperId],
+            [
+                'student_id' => (string) ($record->student_id ?? ''),
+                'teacher_id' => (string) ($record->teacher_id ?? ''),
+                'params' => [
+                    'source' => 'knowledge_explanation',
+                    'knowledge_id' => $paperId,
+                    'kp_codes' => is_array($record->kp_codes) ? $record->kp_codes : [],
+                ],
+                'paper_name' => '知识点讲解_' . $displayCode,
+                'paper_type' => 22,
+                'total_questions' => 0,
+                'total_score' => 0,
+                'status' => 'completed',
+                'difficulty_category' => null,
+                'exam_pdf_url' => $pdfUrl,
+                'grading_pdf_url' => null,
+                'all_pdf_url' => $pdfUrl,
+                'completed_at' => now(),
+            ]
+        );
+    }
 }