Feature/pdf export whisper fallback

[KolaSailaja]

📌 Related Issue

Fixes #870

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🔍 Description

This PR adds PDF export support for chat/session exports and implements a graceful fallback when Whisper is unavailable.

Changes made

• Added pdf to the supported ExportFormat enum.

• Implemented PDF generation for export endpoints.

• Added fallback handling when the Whisper dependency is unavailable:

  • Catches missing Whisper import errors.
  • Logs a warning instead of failing the export.
  • Continues generating a text-only PDF export.

• Updated backend dependencies to include the required PDF generation library.

• Added tests covering PDF export functionality and the Whisper fallback behavior.

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📂 Files Changed

• backend/models/schemas.py
• backend/routes/export.py
• backend/requirements.txt
• backend/requirements_fixed.txt
• backend/tests/test_pdf_export.py

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🧪 Testing

• Verified PDF export generation.
• Verified export succeeds when Whisper is unavailable.
• Verified existing export formats remain unaffected.
• Added automated tests for the new functionality.

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📸 Screenshots

N/A (Backend feature)

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✅ Checklist

• Implemented the requested functionality.
• Added tests for the modified path.
• Preserved existing functionality.
• Followed the project's coding style.
• Kept changes focused on the requested feature.

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