fix: answer null instead of 500 when the export marker is missing

docs/api-consumer-guide.md

@@ -92,7 +92,9 @@ Product master data from the upstream BusProNet system, cached server-side for 1
 
 #### `GET /api/last-update` — scope `api`
 
-Timestamp of the last upstream data export: `{"timestamp": "YYYY-MM-DD HH:MM:SS"}`. Note: if no export marker file exists the current implementation errors (500) rather than returning null — treat a non-JSON/500 response as "unknown".
+Timestamp of the last upstream data export: `{"timestamp": "YYYY-MM-DD HH:MM:SS"}` in `Europe/Berlin` local time.
+If the export marker is missing or unreadable (for instance mid-sync), the answer is `{"timestamp": null}` with
+status 200 — treat null as "unknown", not as "changed".
 
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