feat: show error reason when monitors go DOWN

Propagate check failure reasons through the entire stack:
- Checker captures specific errors (DNS, timeout, HTTP status, SSL, etc.)
- Engine tracks LastError, StatusChangedAt, LastSuccessAt per monitor
- State transitions persisted to new state_changes table
- Detail panel shows error reason, HTTP code, state duration, last
  success time, and last 5 state change events
- Monitor table shows inline error preview for DOWN services
- Alert messages include error reason
- Probe nodes forward error reasons to leader

15 files changed across models, checker, engine, store, TUI, and probes.

internal/monitor/aggregator.go

@@ -11,10 +11,11 @@ const (
 )
 
 type NodeResult struct {
-	NodeID    string
-	IsUp      bool
-	LatencyNs int64
-	CheckedAt time.Time
+	NodeID      string
+	IsUp        bool
+	LatencyNs   int64
+	CheckedAt   time.Time
+	ErrorReason string
 }
 
 func AggregateStatus(results []NodeResult, strategy AggregationStrategy) (isUp bool, avgLatencyNs int64) {