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/store/store.go

@@ -38,6 +38,10 @@ type Store interface {
 	SaveCheckFromNode(siteID int, nodeID string, latencyNs int64, isUp bool) error
 	LoadAllHistory(limit int) (map[int][]models.CheckRecord, error)
 
+	// State Changes
+	SaveStateChange(siteID int, fromStatus, toStatus, errorReason string) error
+	GetStateChanges(siteID int, limit int) ([]models.StateChange, error)
+
 	// Nodes
 	RegisterNode(node models.ProbeNode) error
 	GetNode(id string) (models.ProbeNode, error)