fix(tui,status,store): add delete confirm, input validation, XSS fix, history persistence

Prevent accidental deletes with y/n confirmation dialog. Validate all
numeric form inputs (interval, port, timeout, threshold, retries) with
range checks instead of silently defaulting to zero. Escape user-supplied
data in status page JavaScript to close XSS via monitor names. Persist
check history to new check_history table so sparklines and uptime
percentages survive restarts.

internal/models/models.go

@@ -50,7 +50,13 @@ type User struct {
 	Role      string
 }
 
-// Phase 5: Backup Structure
+type CheckRecord struct {
+	SiteID    int
+	LatencyNs int64
+	IsUp      bool
+	CheckedAt time.Time
+}
+
 type Backup struct {
 	Sites  []Site        `json:"sites"`
 	Alerts []AlertConfig `json:"alerts"`