Add tune_lgbm() and wire hyperparameter tuning into DAG

Converts scratch/tune_model.R into a pure tune_lgbm() function,
replacing hardcoded winning_params with a fully automated tar_target.
Best params (trees=844, depth=3, lr=0.0204, min_n=389) now flow
reproducibly into evaluate_final_model() and train_production_model().
PR-AUC improved from 0.165 to 0.198.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_pkgdown.yml

@@ -34,6 +34,7 @@ reference:
     desc: "Cross-validation and imbalance strategy testing."
     contents:
       - run_imbalance_tournament
+      - tune_lgbm
       - train_diag_model
       - create_efficiency_plot    # Moved here: Belongs with the tournament