perf(metrics): tiered rollup history (7d at ~1.5MB) and cleaner ranges

Replace the flat 48h@2s ring buffer with a 3-tier rollup ladder (2s/1h, 1m/48h, 10m/7d). A sample feeds every tier and rolls up into progressively coarser averages, so per-metric footprint drops from ~21MB to ~1.5MB (measured, 16 system metrics) while extending the range from 48h to 7 days. aggregate() picks the finest tier covering the requested span; a pre-tier flat gob is migrated by replaying its samples through the rollup.

Tidy the dashboard ranges to a professional ladder: 2m, 1h, 3h, 6h, 12h, 24h, 2d, 7d (drop the irregular 2h/5h, the redundant 30m, and the excessive 30d). The allow-list keeps bucket 30 because the node history panel uses it.

Add an initial FreeOSMemory about 60s after boot to reclaim the startup and metric-restore peak instead of waiting for the periodic release. Cover the rollup, tier selection, round-trip, and footprint with tests.

internal/web/web.go

@@ -398,6 +398,10 @@ func (s *Server) startTask(restartXray bool) {
 
 	if mins := sys.MemoryReleaseIntervalMinutes(); mins > 0 {
 		s.cron.AddJob(fmt.Sprintf("@every %dm", mins), job.NewMemoryReleaseJob())
+		go func() {
+			time.Sleep(time.Minute)
+			job.NewMemoryReleaseJob().Run()
+		}()
 	}
 }