Fix PyMuPDF: replace get_text("markdown") with get_text("text")

"markdown" format requires PyMuPDF >= 1.24.0, not available on server.

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

backend/services/docling_service.py

@@ -82,7 +82,7 @@ def _parse_pdf_with_pymupdf(file_path: str) -> str:
     parts = []
     with fitz.open(file_path) as doc:
         for page_num, page in enumerate(doc, 1):
-            text = page.get_text("markdown").strip()
+            text = page.get_text("text").strip()
             if text:
                 parts.append(f"## Page {page_num}\n\n{text}")