docs: clarify default data flow

docs/target-architecture.md

@@ -154,6 +154,8 @@ source of truth for historical queries.
 
 ## Data Flow
 
+Default production flow:
+
 ```mermaid
 flowchart LR
   vehicle["Vehicles / Platforms"] --> ingest["Protocol apps"]
@@ -161,12 +163,14 @@ flowchart LR
   ingest --> kafka["Kafka<br/>event + raw envelopes"]
   kafka --> history["vehicle-history-app"]
   kafka --> analytics["vehicle-analytics-app"]
-  kafka --> state["vehicle-state-service"]
   history --> tdengine["TDengine<br/>raw_frames + locations"]
   analytics --> mysql["MySQL<br/>vehicle_stat_metric"]
-  state --> redisState["Redis<br/>latest state"]
 ```
 
+Optional latest-state flow, enabled only with `-Poptional-latest-state`, consumes
+the same Kafka envelopes and rebuilds Redis snapshots outside the default
+production deployment.
+
 ## Failure Handling
 
 - Protocol apps continue accepting traffic when history, analytics, or Redis