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>

_targets.R

@@ -307,12 +307,7 @@ list(
   ),
   tar_target(
     winning_params,
-    list(
-      trees      = 844,
-      tree_depth = 3,
-      learn_rate = 0.0204,
-      min_n      = 389
-    )
+    tune_lgbm(imbalance_windows)
   ),
   tar_target(
     production_model_uri,