feat: add PDF report generation for backtest executions

- Implemented `generate_backtest_report` to create PDF reports for backtest results.
- Added `report_file` field to `StrategyExecution` for storing report paths.
- Introduced `/executions/<execution_id>/report/` endpoint for downloading reports.
- Enhanced backtesting flow to generate and save reports upon completion.
- Updated dependencies to include `matplotlib` for report generation.

strategies/migrations/0004_strategyexecution_report_file.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.7 on 2026-02-08 05:29
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('strategies', '0003_strategyexecution_callback_response_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='strategyexecution',
+            name='report_file',
+            field=models.CharField(blank=True, help_text='생성된 PDF 리포트 파일 경로', max_length=500),
+        ),
+    ]