feat: record app and APNs environment on user devices

[roncodes]

Summary

Adds three optional, nullable columns to user_devices, and makes them fillable on UserDevice:

Column	Purpose
app_identifier (indexed)	uuid of the app owner (e.g. a Storefront store or network) the token was registered through, so senders pick that app's push credentials first
environment	APNs environment that issued an iOS token (production / sandbox), so it is sent to the right APNs host instead of being rejected with BadDeviceToken
last_seen_at	when the device last re-registered its token

This supports the Storefront push notification fix (fleetbase/storefront fix/push-notification-layer). Storefront only writes these columns when they exist, so the two PRs can ship in either order.

Validation

• The migration guards every column with Schema::hasColumn; down() drops only existing columns.
• php -l passes on both files, and the PHP formatter (php-cs-fixer --dry-run) reports 0 changes.
• No behavior change for existing callers: all columns are nullable, and no existing code reads them.

Risk

Low. This adds nullable columns to user_devices; existing reads and writes are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3965998) to head (5ded447).

Additional details and impacted files

@@             Coverage Diff             @@
##                main      #276   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      7499      7499           
===========================================
  Files            430       430           
  Lines          24488     24488           
===========================================
  Hits           24488     24488           

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

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