Fix: Delete uploaded files on session deletion (#90)

[diksha78dev]

Fixes #90

• Added logic to physically remove the ./data/uploads/{session_id} directory in the DELETE /api/sessions/{session_id} route using shutil.rmtree.
• Wrapped the deletion in a try-except block to gracefully handle missing directories or file locks without crashing the API response.
• Added test_delete_session_removes_files in test_api.py to verify the cleanup behavior.

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