Return native Engine NotFound faults and clear Web UI session on 401.

Include the bad id in host/datacenter/cluster errors, expire stale auth in the
console, and document Ingress annotations that preserve fault bodies.

app/ovirt/common.py

@@ -0,0 +1,43 @@
+"""Shared oVirt Engine helpers (existence checks, native NotFound faults)."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from asyncpg import Connection
+
+from app.ovirt.errors import OVirtError
+
+
+def no_such(kind: str, entity_id: str) -> OVirtError:
+    """Engine-shaped 404 with the bad id in the detail (never an HTML page)."""
+
+    return OVirtError(
+        "NotFound",
+        f"No such {kind} ('{entity_id}')",
+        status_code=404,
+    )
+
+
+async def require_host(conn: Connection, host_id: str) -> Any:
+    row = await conn.fetchrow("SELECT * FROM ov_hosts WHERE id=$1::uuid", host_id)
+    if row is None:
+        raise no_such("host", host_id)
+    return row
+
+
+async def require_datacenter(conn: Connection, datacenter_id: str) -> Any:
+    row = await conn.fetchrow(
+        "SELECT * FROM ov_datacenters WHERE id=$1::uuid",
+        datacenter_id,
+    )
+    if row is None:
+        raise no_such("datacenter", datacenter_id)
+    return row
+
+
+async def require_cluster(conn: Connection, cluster_id: str) -> Any:
+    row = await conn.fetchrow("SELECT * FROM ov_clusters WHERE id=$1::uuid", cluster_id)
+    if row is None:
+        raise no_such("cluster", cluster_id)
+    return row