add Fabric read model rollback activation

Assistant: codex
Assistant-Model: gpt-5.6-sol
Assistant-Session: 01a053ff-1d6f-7fe2-ac1c-a6eb40a42a0c

docs/fabric-graph-read-model.md

@@ -61,6 +61,20 @@ curl -s http://127.0.0.1:8000/fabric/graph-exports/latest
 curl -s http://127.0.0.1:8000/fabric/graph/summary
 ```
 
+Imports are immutable and retained. To roll the read model back to a previously
+validated import without depending on the source service, activate its import
+record; repeat the operation with the newer import id to roll forward again:
+
+```bash
+curl -s -X POST \
+  http://127.0.0.1:8000/fabric/graph-exports/<import-id>/activate \
+  -H "Content-Type: application/json" \
+  -d '{"requested_by":"operator-rollback"}'
+```
+
+Activation only moves the per-source latest marker. It does not alter or delete
+either graph snapshot, and activating the current import is idempotent.
+
 Representative relationship queries:
 
 ```bash