feat(02-01): add config backup env vars, NATS event, device SSH fields, migration, metrics

- Config: CONFIG_BACKUP_INTERVAL (21600s), CONFIG_BACKUP_MAX_CONCURRENT (10), CONFIG_BACKUP_COMMAND_TIMEOUT (60s)
- NATS: ConfigSnapshotEvent type, PublishConfigSnapshot method, config.snapshot.> stream subject
- Device: SSHPort/SSHHostKeyFingerprint fields, UpdateSSHHostKey method, updated queries/scans
- Migration 028: ssh_port, ssh_host_key_fingerprint, timestamp columns with poller_user grants
- Metrics: ConfigBackupTotal (counter), ConfigBackupDuration (histogram), ConfigBackupActive (gauge)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

poller/internal/observability/metrics.go

@@ -58,3 +58,23 @@ var CircuitBreakerResets = promauto.NewCounter(prometheus.CounterOpts{
 	Name: "mikrotik_circuit_breaker_resets_total",
 	Help: "Total circuit breaker resets when a device recovers.",
 })
+
+// ConfigBackupTotal counts config backup operations by status.
+// Status labels: "success", "error", "skipped_offline", "skipped_auth_blocked", "skipped_hostkey_blocked".
+var ConfigBackupTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+	Name: "mikrotik_config_backup_total",
+	Help: "Total config backup operations.",
+}, []string{"status"})
+
+// ConfigBackupDuration tracks the duration of individual config backup operations.
+var ConfigBackupDuration = promauto.NewHistogram(prometheus.HistogramOpts{
+	Name:    "mikrotik_config_backup_duration_seconds",
+	Help:    "Duration of a single config backup operation in seconds.",
+	Buckets: []float64{1, 5, 10, 30, 60, 120, 300},
+})
+
+// ConfigBackupActive tracks the number of concurrent config backup jobs running.
+var ConfigBackupActive = promauto.NewGauge(prometheus.GaugeOpts{
+	Name: "mikrotik_config_backup_active",
+	Help: "Number of concurrent config backup jobs running.",
+})