add Fabric read model rollback activation

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

api/services/fabric_graph.py

@@ -221,6 +221,55 @@ async def ingest_fabric_graph_export(
     return import_run, True, False
 
 
+async def activate_fabric_graph_import(
+    session: AsyncSession,
+    import_id: Any,
+    *,
+    requested_by: str,
+) -> tuple[FabricGraphImport | None, Any | None, bool]:
+    """Select a retained valid import as the active read model.
+
+    Imports are immutable. Activation only moves the per-source ``is_latest``
+    marker, which makes cutover rollback fast and avoids re-fetching an older
+    authority payload during an incident.
+    """
+    result = await session.execute(
+        select(FabricGraphImport).where(FabricGraphImport.id == import_id)
+    )
+    import_run = result.scalar_one_or_none()
+    if import_run is None:
+        return None, None, False
+    if import_run.validation_status != "valid":
+        raise ValueError("Only a valid Fabric graph import can be activated.")
+
+    previous_result = await session.execute(
+        select(FabricGraphImport).where(
+            FabricGraphImport.source_repo_slug == import_run.source_repo_slug,
+            FabricGraphImport.is_latest.is_(True),
+        )
+    )
+    previous = previous_result.scalars().first()
+    if previous is not None and previous.id == import_run.id:
+        return import_run, previous.id, False
+
+    await _mark_latest(session, import_run)
+    import_run.last_seen_at = datetime.now(timezone.utc)
+    await _record_progress(
+        session,
+        "Fabric graph read model activation changed.",
+        {
+            "source_repo_slug": import_run.source_repo_slug,
+            "import_id": str(import_run.id),
+            "previous_import_id": str(previous.id) if previous else None,
+            "content_hash": import_run.content_hash,
+            "requested_by": requested_by,
+        },
+    )
+    await session.commit()
+    await session.refresh(import_run)
+    return import_run, previous.id if previous else None, True
+
+
 def validate_fabric_graph_export(payload: dict[str, Any]) -> FabricGraphExportPayload:
     try:
         export = FabricGraphExportPayload.model_validate(payload)