feat: add PDF export generation via WeasyPrint with branded template

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/sessions.py

@@ -391,6 +391,22 @@ async def export_session(
             detail="You don't have access to this session"
         )
 
+    # PDF export — separate path with binary response
+    if export_options.format == "pdf":
+        from app.services.export_service import generate_pdf_export
+        from fastapi.responses import Response
+        pdf_bytes = await generate_pdf_export(session, export_options, db)
+
+        if session.completed_at:
+            session.exported = True
+            await db.commit()
+
+        return Response(
+            content=pdf_bytes,
+            media_type="application/pdf",
+            headers={"Content-Disposition": f'attachment; filename="session-export-{session_id}.pdf"'},
+        )
+
     # Generate export based on format
     if export_options.format == "markdown":
         content = generate_markdown_export(session, export_options)