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/store/sqlite.go

@@ -57,7 +57,15 @@ func (s *SQLiteStore) Init() error {
 		username TEXT NOT NULL,
 		public_key TEXT NOT NULL,
 		role TEXT DEFAULT 'user'
-	);`
+	);
+	CREATE TABLE IF NOT EXISTS check_history (
+		id INTEGER PRIMARY KEY AUTOINCREMENT,
+		site_id INTEGER NOT NULL,
+		latency_ns INTEGER,
+		is_up BOOLEAN,
+		checked_at DATETIME DEFAULT CURRENT_TIMESTAMP
+	);
+	CREATE INDEX IF NOT EXISTS idx_check_history_site ON check_history(site_id, checked_at DESC);`
 	_, err = s.db.Exec(createTables)
 	if err != nil {
 		return err
@@ -204,7 +212,38 @@ func (s *SQLiteStore) DeleteUser(id int) error {
 	return err
 }
 
-// --- PHASE 5 ---
+func (s *SQLiteStore) SaveCheck(siteID int, latencyNs int64, isUp bool) {
+	s.db.Exec("INSERT INTO check_history (site_id, latency_ns, is_up) VALUES (?, ?, ?)", siteID, latencyNs, isUp)
+	s.db.Exec(`DELETE FROM check_history WHERE site_id = ? AND id NOT IN (
+		SELECT id FROM check_history WHERE site_id = ? ORDER BY checked_at DESC LIMIT 1000
+	)`, siteID, siteID)
+}
+
+func (s *SQLiteStore) LoadAllHistory(limit int) map[int][]models.CheckRecord {
+	result := make(map[int][]models.CheckRecord)
+	rows, err := s.db.Query(`
+		SELECT site_id, latency_ns, is_up FROM (
+			SELECT site_id, latency_ns, is_up,
+				ROW_NUMBER() OVER (PARTITION BY site_id ORDER BY checked_at DESC) AS rn
+			FROM check_history
+		) WHERE rn <= ?`, limit)
+	if err != nil {
+		return result
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var r models.CheckRecord
+		rows.Scan(&r.SiteID, &r.LatencyNs, &r.IsUp)
+		result[r.SiteID] = append(result[r.SiteID], r)
+	}
+	for id, records := range result {
+		for i, j := 0, len(records)-1; i < j; i, j = i+1, j-1 {
+			records[i], records[j] = records[j], records[i]
+		}
+		result[id] = records
+	}
+	return result
+}
 
 func (s *SQLiteStore) ExportData() models.Backup {
 	return models.Backup{