fix(dialog): use uuid.New() for ticket and audit IDs

- Replace fmt.Sprintf with sess.ID+nanotime that generated non-UUID strings
- ticket creation and audit logging now use github.com/google/uuid
- Fixes 500 error when webhook processes messages with PG store
- All 23/23 tests pass, verified Gate B end-to-end

go.mod

@@ -2,4 +2,7 @@ module github.com/bridge/ai-customer-service
 
 go 1.22
 
-require github.com/lib/pq v1.10.9
+require (
+	github.com/google/uuid v1.6.0
+	github.com/lib/pq v1.10.9
+)