feat(pokemon): enforce bidirectional opposite sync and hide opposite text

Add unique indexes and transactional sync for opposite configurations
Remove explicit opposite names and labels from Pokemon detail view

backend/db/schema.sql

@@ -1575,6 +1575,14 @@ BEGIN
   END IF;
 END $$;
 
+CREATE UNIQUE INDEX IF NOT EXISTS environments_opposite_environment_unique_idx
+  ON environments(opposite_environment_id)
+  WHERE opposite_environment_id IS NOT NULL;
+
+CREATE UNIQUE INDEX IF NOT EXISTS favorite_things_opposite_favorite_thing_unique_idx
+  ON favorite_things(opposite_favorite_thing_id)
+  WHERE opposite_favorite_thing_id IS NOT NULL;
+
 UPDATE items
 SET dyeability = CASE
   WHEN dual_dyeable THEN 2