CUB-230: hub-side deduplication for offline buffering replay

- Add dedup check in handleStatus using (camera_id, recorded_at) uniqueness
- Skip insert if duplicate detected - logs replayed entries
- Go mod: updated version to 1.19

internal/mqtt/subscriber.go

@@ -209,6 +209,21 @@ func (s *Subscriber) handleStatus(cameraID string, payload []byte) {
 		prevRecording = -1 // no previous status
 	}
 
+	// CUB-230: Deduplication check - skip if same (camera_id, recorded_at) exists
+	// This handles replayed entries from offline buffering
+	var dupCount int
+	err = s.db.QueryRow(`
+		SELECT COUNT(*) FROM status_logs
+		WHERE camera_id = ? AND recorded_at = ?
+	`, cameraID, ts).Scan(&dupCount)
+	if err != nil {
+		log.Printf("MQTT status dedup check error for %s: %v", cameraID, err)
+		// Continue anyway if check fails
+	} else if dupCount > 0 {
+		log.Printf("MQTT status deduplicated (camera_id=%s, recorded_at=%s) - replay from offline buffer", cameraID, ts.Format("2006-01-02 15:04:05"))
+		return
+	}
+
 	// Insert status_log
 	_, err = s.db.Exec(`
 		INSERT INTO status_logs (camera_id, recorded_at, battery_pct,