Fix Jinja2 template error in predict.html

Fixed UndefinedError caused by complex Jinja2 expression when calculating
Year 10 averages. Moved the average calculation to Python backend for better
maintainability and performance.

Changes:
- Calculate year10_averages in the predict() route
- Pass averages to template as a simple dictionary
- Simplified template logic to use direct dictionary lookup

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