Fix/pdf export whisper fallback

[KolaSailaja]

📌 Summary

Closes #870

This PR adds a graceful fallback for PDF export when the optional Whisper dependency is unavailable.

What Changed

• Added a fallback path so PDF export continues to work when Whisper is not installed or cannot be imported.
• Improved the export flow to handle optional Whisper dependency failures without breaking the export process.
• Added the required database helper updates to support the export workflow.
• Preserved existing behavior when Whisper is available.
• Kept the implementation localized to minimize merge conflicts and maintain backward compatibility.

Testing

The following tests were executed successfully:

• ✅ tests/test_pdf_export.py
• ✅ tests/test_bulk_export.py
• ✅ tests/test_api.py

Result:

• 18 tests passed

The implementation was also validated for both scenarios:

• Whisper available
• Whisper unavailable (fallback path)

Files Updated

• backend/routes/export.py
• backend/services/db_service.py
• backend/tests/test_pdf_export.py (and any related test updates)

Checklist

• Implements the requested fallback behavior
• Preserves backward compatibility
• Adds/updates tests for the modified functionality
• Keeps the implementation focused and production-ready
• No unrelated refactoring
• Ready for review

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