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.

pyproject.toml

@@ -8,4 +8,5 @@ dependencies = [
     "yfinance>=0.2.66",
     "gunicorn>=21.2.0",
     "requests>=2.31.0",
+    "matplotlib>=3.9.0",
 ]