Claude/issue 334 system message payload

apps/backend/drizzle/0002_strengthen_system_payload_check.sql

@@ -0,0 +1,29 @@
+-- Backfill: #398 added `system_payload` but no producer ever wrote to it, so
+-- every pre-existing system row still has its {userId, change} JSON sitting in
+-- `ciphertext` with `system_payload` NULL. The stricter constraint below would
+-- reject those rows outright, so move the data across first. Unparseable
+-- values are preserved under a `legacyCiphertext` key rather than dropped.
+DO $$
+DECLARE
+  row RECORD;
+BEGIN
+  FOR row IN
+    SELECT id, ciphertext FROM "messages"
+    WHERE content_type = 'system' AND system_payload IS NULL
+  LOOP
+    BEGIN
+      UPDATE "messages"
+      SET system_payload = row.ciphertext::jsonb,
+          ciphertext = NULL
+      WHERE id = row.id;
+    EXCEPTION WHEN OTHERS THEN
+      UPDATE "messages"
+      SET system_payload = jsonb_build_object('legacyCiphertext', row.ciphertext),
+          ciphertext = NULL
+      WHERE id = row.id;
+    END;
+  END LOOP;
+END $$;
+--> statement-breakpoint
+ALTER TABLE "messages" DROP CONSTRAINT "messages_system_payload_only_on_system_type";--> statement-breakpoint
+ALTER TABLE "messages" ADD CONSTRAINT "messages_system_payload_check" CHECK ("messages"."content_type" <> 'system' OR ("messages"."ciphertext" IS NULL AND "messages"."system_payload" IS NOT NULL));