Replace magic number 9999 with named constant in balancer/core.py

[alfieprojectsdev]

This change replaces the magic number 9999 (used as a placeholder for infinite stock duration when burn rate is zero) with a named constant INFINITE_DOI.

Changes:

1. In balancer/core.py, defined INFINITE_DOI = 9999.0.
2. Updated calculate_bmi to return INFINITE_DOI instead of 9999.0 when daily_burn_rate <= 0.
3. Updated detect_imbalances to use INFINITE_DOI instead of 9999 when calculating days_to_consume_locally for items with zero burn rate.
4. Added a new test case test_zero_burn_rate to balancer/test_balancer.py to ensure that both calculate_bmi and the surplus detection logic correctly handle zero burn rates using the new constant.

All tests passed. Build artifacts (like __pycache__) have been removed.

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PR created automatically by Jules for task 5114800275831407095 started by @alfieprojectsdev

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