Fix multi-page PDF upload: include session_id for first page

The frontend expects session_id in the upload response, but multi-page
PDFs returned only document_group_id + pages[]. Now includes session_id
pointing to the first page for backwards compatibility.

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

klausur-service/backend/ocr_pipeline_sessions.py

@@ -222,7 +222,12 @@ async def _create_multi_page_sessions(
             f"(page {page_idx + 1}/{page_count}) from {filename} ({w}x{h})"
         )
 
+    # Include session_id pointing to first page for backwards compatibility
+    # (frontends that expect a single session_id will navigate to page 1)
+    first_session_id = pages[0]["session_id"] if pages else None
+
     return {
+        "session_id": first_session_id,
         "document_group_id": document_group_id,
         "filename": filename,
         "name": base_name,