fix(dashboard): measure V8 heap against its limit, not its current size

The Health Score's heap component divided heapUsed by heapTotal, which is
only what V8 has grown the heap to so far, so a healthy server read 70-90%
and dragged the score down. GET /api/settings/info now also reports
server.memory.heapLimit (v8 heap_size_limit, the ceiling the process dies
at); the V8 Heap bar and the Health Score divide by it, falling back to
heapTotal against an older server.

server/openapi-extra/override-ops.js

@@ -139,6 +139,7 @@ const SETTINGS_INFO_EXAMPLE = {
       heapTotal: 71303168,
       heapUsed: 58392104,
       external: 3211884,
+      heapLimit: 4345298944,
       arrayBuffers: 1048576,
     },
     cpu_load: [2.13, 2.45, 2.31],