improve the notify mechanism for resource change

backend/migrations/20250902085503_notify_resource_cache_on_variable_change.down.sql

@@ -0,0 +1 @@
+-- Add down migration script here

backend/migrations/20250902085503_notify_resource_cache_on_variable_change.up.sql

@@ -0,0 +1,23 @@
+-- Add up migration script here
+CREATE OR REPLACE FUNCTION notify_var_resource_cache_change()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF TG_TABLE_NAME = 'variable' THEN
+    PERFORM pg_notify('var_cache_invalidation', 
+        json_build_object(
+            'workspace_id', COALESCE(NEW.workspace_id, OLD.workspace_id),
+            'path', COALESCE(NEW.path, OLD.path),
+            'operation', TG_OP
+        )::text
+    );
+    END IF;
+    PERFORM pg_notify('resource_cache_invalidation',
+        json_build_object(
+            'workspace_id', COALESCE(NEW.workspace_id, OLD.workspace_id),
+            'path', COALESCE(NEW.path, OLD.path),
+            'operation', TG_OP
+        )::text
+    );
+    RETURN COALESCE(NEW, OLD);
+END;
+$$ LANGUAGE plpgsql;