add guard clause when backfilling plan_id to recommendation

src/app/db/migrations/0223_recommendation_plan_id_backfill.sql

@@ -1,3 +1,21 @@
+-- Guard: fail if any recommendation is linked to more than one plan.
+-- The spec asserts each recommendation belongs to exactly one plan; if real
+-- data violates that the backfill cannot pick deterministically and must not
+-- proceed silently.
+DO $$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM   plan_recommendations
+    GROUP  BY recommendation_id
+    HAVING count(*) > 1
+  ) THEN
+    RAISE EXCEPTION
+      'plan_id backfill aborted: one or more recommendations appear in multiple '
+      'plan_recommendations rows. Resolve cardinality before re-running.';
+  END IF;
+END $$;
+
 UPDATE recommendation r
 SET    plan_id = pr.plan_id
 FROM   plan_recommendations pr