Add question bank mastery paths and gap questions

ENVIRONMENTS.md

@@ -77,6 +77,24 @@ Promotion is idempotent and does not delete or implicitly unpublish PRD content.
 
 ## Verification and rollback
 
+## Step 3 mastery system backfill
+
+The mastery dashboard uses a canonical 14-system classification stored on each
+question. After deploying the mastery-capable API, inspect and apply the
+idempotent backfill in UAT:
+
+```bash
+kubectl exec deployment/step3-api -n step3-uat -- \
+  dotnet QuestionsApi.dll backfill-step3-systems
+kubectl exec deployment/step3-api -n step3-uat -- \
+  dotnet QuestionsApi.dll backfill-step3-systems --apply --confirm BACKFILL_STEP3_SYSTEMS
+```
+
+Review the reported unmapped count before acceptance testing. Repeat the dry run
+and confirmed apply in `step3-prd` after the tested revision is promoted to the
+production branch. The command is safe to rerun and does not modify attempts or
+user data.
+
 ## Rz Coin Store attempt-cycle migration
 
 The QBank reset preserves every attempt and increments a user's eligibility cycle. After deploying