Surface policy-engine errors from stored-query invoke

The invoke handler mapped every authorize_request failure to 404 ('stored
query not found'), which collapsed the authorization decision (deny -> 403)
together with operational failures (no actor -> 401, Cedar evaluation error ->
500). A real policy-engine 500 was hidden as a missing query.

Separate the two concerns instead of sniffing the masked status. Extract
authorize() returning an Authz { Allowed, Denied(msg) } decision and reserve
Err for operational failures only; authorize_request becomes a thin wrapper
that maps Denied -> 403, so the 16 deny-as-403 callers are unchanged. The
invoke handler now matches the decision directly: a denial stays 404 (deny ==
missing, so the catalog can't be probed without the grant), while a 401/500
propagates with its true status.

500 is now a reachable outcome on POST /queries/{name}; document it in the
endpoint responses and regenerate openapi.json.

openapi.json

@@ -975,6 +975,16 @@
                 }
               }
             }
+          },
+          "500": {
+            "description": "Policy evaluation error (a denial is reported as 404, not 500)",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ErrorOutput"
+                }
+              }
+            }
           }
         },
         "security": [