fix(metrics): host memory usage excludes reclaimable page cache (#1297)

* fix(metrics): host memory usage excludes reclaimable page cache

Host RAM was computed as mem.used / mem.total via systeminformation, but mem.used counts
reclaimable buffers/cache as used, so a busy Linux host read ~99%. Switch the dashboard stats,
the fleet node self-report, and the host-RAM alert threshold to mem.active / mem.total
(cache-excluded), and report used: mem.active and free: mem.available so the byte readout stays
consistent with the percentage. Add regression tests for a cache-heavy host (no false alert) and
a genuinely busy host (alert still fires).

* test(metrics): assert system stats memory excludes reclaimable cache

The cached /api/system/stats test mocked si.mem() without active/available, so after
the route switched to the cache-excluded fields it produced a NaN percentage that the
shape-only assertions did not catch. Fill the mock with a realistic shape and assert the
route reports the active working set, not the cache-inclusive used/free.

backend/src/services/MonitorService.ts

@@ -299,7 +299,10 @@ export class MonitorService {
                 this.clearHostMetricSuppression('cpu');
             }
 
-            const ramUsage = (mem.used / mem.total) * 100;
+            // Key off mem.active (the real working set), not mem.used: on Linux/BSD/macOS
+            // mem.used counts reclaimable page cache and reads ~99% on a busy host, which
+            // would fire spurious host-memory alerts.
+            const ramUsage = (mem.active / mem.total) * 100;
             const ramLimit = parseFloat(settings['host_ram_limit']);
             if (!isNaN(ramLimit) && ramLimit > 0 && ramUsage > ramLimit) {
                 await this.dispatchHostMetricAlert('ram', 'warning', suppressionMs,