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/models/models.go

@@ -27,14 +27,26 @@ type Site struct {
 	Paused         bool
 	Regions        string
 
-	FailureCount   int
-	Status         string
-	StatusCode     int
-	Latency        time.Duration
-	CertExpiry     time.Time
-	HasSSL         bool
-	LastCheck      time.Time
-	SentSSLWarning bool
+	FailureCount    int
+	Status          string
+	StatusCode      int
+	Latency         time.Duration
+	CertExpiry      time.Time
+	HasSSL          bool
+	LastCheck       time.Time
+	SentSSLWarning  bool
+	LastError       string
+	StatusChangedAt time.Time
+	LastSuccessAt   time.Time
+}
+
+type StateChange struct {
+	ID          int
+	SiteID      int
+	FromStatus  string
+	ToStatus    string
+	ErrorReason string
+	ChangedAt   time.Time
 }
 
 type AlertConfig struct {