test(reports): add ReportEngine unit tests [FASE-2.1]

Stub WeasyPrint for CI-friendly PDF generation and verify HTML render, PDF path, and HTML file output.

backend/app/services/report_engine.py

@@ -76,6 +76,10 @@ class ReportEngine:
         logger.info("DOCX generated: %s", output_path)
         return output_path
 
+    def generate_html(self, template_name: str, context: dict) -> str:
+        """Render and save a standalone HTML report (alias for spec compatibility)."""
+        return self.generate_html_file(template_name, context)
+
     def generate_html_file(self, template_name: str, context: dict) -> str:
         """Render and save a standalone HTML report."""
         html_content = self.render_html(template_name, context)