fix(push): send FCM order notifications with android high priority

[roncodes]

Fixes #268.

Why

PushNotification::createFcmMessage() built FCM messages without an Android priority. Android held order pushes (OrderAssigned, OrderDispatched, …) while a driver's phone was in deep Doze, and delivered them only once the phone was unlocked or charged. FCM still accepted the message, so nothing on the server reported a problem.

The reporter ran a controlled test on the same device after about 60 minutes locked: the message as built today was not delivered, and the same message with android.priority = high arrived immediately.

What changed

• createFcmMessage() sets android.priority = high. These are user-visible, time-sensitive notifications, which is the case FCM documents high priority for.
• APNs is unchanged; alert pushes already default to apns-priority: 10.

Verification

• The FCM message shape tests (PushNotificationTest, NotificationsAndMailTest) now expect priority: high.

Storefront builds its own FCM messages (storefront/server/src/Support/PushNotification.php), which probably needs the same change in that repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3965998) to head (68b1938).

Additional details and impacted files

@@             Coverage Diff             @@
##                main      #273   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      7499      7499           
===========================================
  Files            430       430           
  Lines          24488     24489    +1     
===========================================
+ Hits           24488     24489    +1     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)

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