Convert print statements to proper logging for systemd capture

- Convert all print statements in process_pdfs_async to logger.info/logger.error
- Update startup diagnostics to use logger.info instead of print
- Ensure all diagnostic information is captured by systemd logs on remote server
- Add detailed logging for PDF processing steps, file scanning, and error conditions

requirements.txt

@@ -4,4 +4,4 @@ Werkzeug==3.0.1
 waitress==3.0.0
 requests-oauthlib==2.0.0
 python-dotenv==1.2.1
-# PyMuPDF==1.23.7  # Optional - will be installed separately if possible
+PyMuPDF==1.23.7  # Optional - will be installed separately if possible