Refactor bucket structure: baf-fraud/ prefix under lake bucket

All functions now default to bucket_name = "lake" with "baf-fraud/"
prepended to all layer prefixes, matching the contemporary lakehouse
naming convention (one bucket per environment, project as prefix).

Migration: copy baf-fraud/ data to lake/baf-fraud/ on analyticsvm,
update BAF_BUCKET env var from "baf-fraud" to "lake".

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

man/tune_lgbm.Rd

@@ -6,8 +6,8 @@
 \usage{
 tune_lgbm(
   imbalance_windows,
-  bucket_name = "baf-fraud",
-  inputs_prefix = "05_model_input",
+  bucket_name = "lake",
+  inputs_prefix = "baf-fraud/05_model_input",
   grid_size = 30L,
   seed = 42L
 )
@@ -17,7 +17,7 @@ tune_lgbm(
 \code{train_months}, and \code{test_month}, as produced by the
 \code{imbalance_windows} target.}
 
-\item{bucket_name}{Character. MinIO bucket name. Default \code{"baf-fraud"}.}
+\item{bucket_name}{Character. MinIO bucket name. Default \code{"lake"}.}
 
 \item{inputs_prefix}{Character. Prefix for the model input layer.
 Default \code{"05_model_input"}.}