Closes#74
Adds app/services/formula_engine.py with three pure formula functions
(compute_batter_value, compute_sp_value, compute_rp_value), a dispatch
helper (compute_value_for_track), and a tier classifier (tier_from_value).
Tier boundaries and thresholds match the locked seed data from WP-03.
Note: pitcher formulas use stats.k (not stats.so) to match the
PlayerSeasonStats model field name introduced in WP-02.
19 unit tests in tests/test_formula_engine.py — all pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
