hub: emit battery_pct/video_remaining as null, not omitted

The SPA types these as number|null and null-checks them, but omitempty
dropped the field entirely when uncalibrated → undefined in JS → "NaN%".
Always serialize the field (null when unknown) so the card shows "N/A".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pkg/models/camera.go

@@ -33,8 +33,11 @@ type StatusLog struct {
 type CameraStatus struct {
 	CameraID          string    `json:"camera_id"`
 	FriendlyName      string    `json:"friendly_name"`
-	BatteryPct        *int      `json:"battery_pct,omitempty"`
-	VideoRemainingSec *int      `json:"video_remaining_sec,omitempty"`
+	// Not omitempty: the SPA expects these as `number | null`. Omitting them
+	// makes the field `undefined` in JS, which slips past null checks and
+	// renders as "NaN%".
+	BatteryPct        *int      `json:"battery_pct"`
+	VideoRemainingSec *int      `json:"video_remaining_sec"`
 	Recording         bool      `json:"recording"`
 	Mode              string    `json:"mode"`
 	Resolution        string    `json:"resolution"`