feat: add step cutoff and mid-session export support

- max_step_index slices decisions in all 4 export formats
- Only set exported=True when session is completed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/sessions.py

@@ -314,9 +314,10 @@ async def export_session(
         from app.services.variable_service import resolve_variables
         content = resolve_variables(content, session_vars)
 
-    # Mark as exported
-    session.exported = True
-    await db.commit()
+    # Only mark as exported if session is completed
+    if session.completed_at:
+        session.exported = True
+        await db.commit()
 
     return PlainTextResponse(content=content, media_type=media_type)