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/routes/fleet.ts

@@ -233,9 +233,12 @@ async function fetchLocalNodeOverview(node: Node): Promise<FleetNodeOverview> {
         cpu: { usage: currentLoad.currentLoad.toFixed(1), cores: currentLoad.cpus.length },
         memory: {
           total: mem.total,
-          used: mem.used,
-          free: mem.free,
-          usagePercent: ((mem.used / mem.total) * 100).toFixed(1),
+          // Percentage is keyed 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. mem.available is total - active, so used + free = total.
+          used: mem.active,
+          free: mem.available,
+          usagePercent: ((mem.active / mem.total) * 100).toFixed(1),
         },
         disk: mainDisk ? {
           total: mainDisk.size,

backend/src/routes/metrics.ts

@@ -311,9 +311,12 @@ metricsRouter.get('/system/stats', authMiddleware, async (req: Request, res: Res
           },
           memory: {
             total: mem.total,
-            used: mem.used,
-            free: mem.free,
-            usagePercent: ((mem.used / mem.total) * 100).toFixed(1),
+            // Percentage is keyed 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. mem.available is total - active, so used + free = total.
+            used: mem.active,
+            free: mem.available,
+            usagePercent: ((mem.active / mem.total) * 100).toFixed(1),
           },
           disk: mainDisk ? {
             fs: mainDisk.fs,