Reduce conversion memory footprint

app/services/storage.py

@@ -3,7 +3,7 @@ import uuid
 from dataclasses import asdict, dataclass
 from pathlib import Path
 
-from app.services.exporter import ExportResult
+from app.services.exporter import ExportResult, write_export
 
 
 @dataclass(slots=True)
@@ -14,16 +14,30 @@ class ResultMetadata:
     mimetype: str
 
 
-def persist_result(output_dir: Path, export_result: ExportResult) -> ResultMetadata:
+def persist_result(
+    output_dir: Path,
+    records: list[dict[str, str]],
+    union_keys: list[str],
+    mode: str,
+    output_format: str,
+    preview_record_limit: int,
+) -> tuple[ResultMetadata, ExportResult]:
     """Persist generated output and sidecar metadata in a temporary directory."""
     result_id = uuid.uuid4().hex
-    extension = "txt" if export_result.output_format == "text" else "csv"
+    extension = "txt" if output_format == "text" else "csv"
     mimetype = "text/plain; charset=utf-8" if extension == "txt" else "text/csv; charset=utf-8"
 
     file_path = output_dir / f"{result_id}.{extension}"
     metadata_path = output_dir / f"{result_id}.json"
 
-    file_path.write_text(export_result.content, encoding="utf-8")
+    export_result = write_export(
+        file_path=file_path,
+        records=records,
+        union_keys=union_keys,
+        mode=mode,
+        output_format=output_format,
+        preview_record_limit=preview_record_limit,
+    )
     metadata = ResultMetadata(
         result_id=result_id,
         file_path=str(file_path),
@@ -31,7 +45,7 @@ def persist_result(output_dir: Path, export_result: ExportResult) -> ResultMetad
         mimetype=mimetype,
     )
     metadata_path.write_text(json.dumps(asdict(metadata)), encoding="utf-8")
-    return metadata
+    return metadata, export_result
 
 
 def load_result_metadata(output_dir: Path, result_id: str) -> dict[str, str] | None: