feat: event ids, outbox identity migration

crates/event-payload/src/lib.rs

@@ -154,6 +154,16 @@ pub enum EventPayload {
     },
 }
 
+/// Wire form of an event: the tagged payload plus its optional identity.
+/// Messages without `event_id` (queued before identities existed) still decode.
+#[derive(Debug, Serialize, Deserialize, PartialEq)]
+pub struct EventMessage {
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub event_id: Option<String>,
+    #[serde(flatten)]
+    pub payload: EventPayload,
+}
+
 impl EventPayload {
     pub fn event_type(&self) -> &'static str {
         match self {