refactor(chunks): order chunk reads by (document_id, position)

Presentation and citation ordering moves off Chunk.id/created_at to the
explicit position column (id kept as tiebreaker). Vector and ts_rank
ranking order_by clauses are untouched.

surfsense_backend/app/services/ai_file_sort_service.py

@@ -156,7 +156,7 @@ async def _resolve_document_text(
     stmt = (
         select(Chunk.content)
         .where(Chunk.document_id == document.id)
-        .order_by(Chunk.id)
+        .order_by(Chunk.position, Chunk.id)
         .limit(_MAX_CHUNKS_FOR_CONTEXT)
     )
     result = await session.execute(stmt)

surfsense_backend/app/services/export_service.py

@@ -62,7 +62,7 @@ async def _get_document_markdown(
     chunk_result = await session.execute(
         select(Chunk.content)
         .filter(Chunk.document_id == document.id)
-        .order_by(Chunk.id)
+        .order_by(Chunk.position, Chunk.id)
     )
     chunks = chunk_result.scalars().all()
     if chunks: