refactor: extract magic numbers to named constants

Replace inline numeric literals with named constants across 14 files:
server timeouts/rate limits, cluster thresholds/intervals, DB pool
sizes, alert/dial timeouts, TUI uptime thresholds, node status
thresholds, and state history limits.

internal/tui/const.go

@@ -0,0 +1,20 @@
+package tui
+
+import "time"
+
+const (
+	nodeOnlineThreshold = 60 * time.Second
+	nodeStaleThreshold  = 5 * time.Minute
+
+	uptimeGoodPct      = 99.0
+	uptimeExcellentPct = 99.9
+	uptimeWarnPct      = 95.0
+	uptimePrecisionPct = 99.99
+
+	stateHistoryLookback = 30 * 24 * time.Hour
+	stateHistoryDays     = 30
+	stateHistoryLimit    = 100
+
+	httpErrorThreshold = 400
+	errorDetailMaxLen  = 30
+)