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/service/server.go

@@ -166,15 +166,15 @@ const xrayVersionsCacheTTL = 15 * time.Minute
 // callers from triggering arbitrary aggregation work and keeps the
 // frontend's bucket selector self-documenting.
 var allowedHistoryBuckets = map[int]bool{
-	2:    true, // Real-time view
-	30:   true, // 30s intervals
-	60:   true, // 1m intervals
-	120:  true, // 2m intervals
-	180:  true, // 3m intervals
-	300:  true, // 5m intervals
-	720:  true, // 12m intervals
-	1440: true, // 24m intervals
-	2880: true, // 48m intervals
+	2:     true, // 2m
+	30:    true, // 30m
+	60:    true, // 1h
+	180:   true, // 3h
+	360:   true, // 6h
+	720:   true, // 12h
+	1440:  true, // 24h
+	2880:  true, // 2d
+	10080: true, // 7d
 }
 
 // IsAllowedHistoryBucket reports whether a bucket-seconds value is in the