fix: preserve Chatwoot message visibility and identity

migrations/010_chatwoot_ingestion_identity.sql

@@ -0,0 +1,4 @@
+-- Canonical message identity for replay-safe Chatwoot ingestion.
+CREATE UNIQUE INDEX IF NOT EXISTS ux_messages_source_event
+ON messages(source_system, source_event_id)
+WHERE source_event_id IS NOT NULL;