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>

_targets.R

@@ -32,9 +32,9 @@ list(
   tar_target(
     baf_parquet_prefix,
     convert_to_parquet(
-      from_prefix = "01_raw",
-      to_prefix   = "02_intermediate",
-      bucket_name = "baf-fraud"
+      from_prefix = "baf-fraud/01_raw",
+      to_prefix   = "baf-fraud/02_intermediate",
+      bucket_name = "lake"
     )
   ),
 
@@ -42,8 +42,8 @@ list(
     baf_primary_prefix,
     clean_baf_base(
       in_prefix   = baf_parquet_prefix,
-      out_prefix  = "03_primary/variant=Base",
-      bucket_name = "baf-fraud",
+      out_prefix  = "baf-fraud/03_primary/variant=Base",
+      bucket_name = "lake",
       partitioning = "month",
       existing_data_behavior = "delete_matching",
       verbose = TRUE
@@ -54,8 +54,8 @@ list(
     baf_feature_prefix,
     engineer_features(
       in_prefix = baf_primary_prefix,
-      out_prefix = "04_feature/variant=Base",
-      bucket_name = "baf-fraud",
+      out_prefix = "baf-fraud/04_feature/variant=Base",
+      bucket_name = "lake",
       partitioning = "month",
       existing_data_behavior = "delete_matching",
       verbose = TRUE
@@ -67,8 +67,8 @@ list(
     baf_model_input_prefix,
     generate_model_inputs(
       feature_prefix = baf_feature_prefix,
-      out_prefix = "05_model_input",
-      bucket_name = "baf-fraud"
+      out_prefix = "baf-fraud/05_model_input",
+      bucket_name = "lake"
     )
   ),
 
@@ -104,7 +104,7 @@ list(
   # ---- Fraud Prevalence ----
   tar_target(
     fig_fraud_by_month,
-    plot_fraud_by_month(baf_primary_prefix, bucket_name = "baf-fraud")
+    plot_fraud_by_month(baf_primary_prefix, bucket_name = "lake")
   ),
 
   tar_target(