Enable APNs push provider configuration and registration

Summary

The APNs (Apple Push Notification service) provider is documented but not enabled on this instance. iOS apps that try to register for push notifications cannot do so. The API contract already exists per the Web Push docs, but provider configuration is missing.

Steps to Reproduce

1. GET /push.json - response lists only the webpush provider:

   { "publicKey": "...", "providers": { "webpush": { "publicKey": "..." } } }
   

2. POST /push.json with a valid APNs body:

   { "provider": "apns", "token": "00112233445566778899aabbccddeeff..." }
   

3. Response: 400 with { "error": "Unknown provider" }.

Expected Behaviour

• An administrator should be able to configure the APNs provider (Apple Push Notification service credentials - team/account config, and the .p8/.p12 key or certificate) in the admin settings, matching how the webpush VAPID key is configured.
• Once configured, GET /push.json should list apns under providers, and POST /push.json with { "provider": "apns", "token": "<hex device token>" } should register the device and send the welcome notification.

Actual Behaviour

• APNs registrations fail with HTTP 400 and { "error": "Unknown provider" }.
• GET /push.json lists only the webpush provider.
• The push page states: "apns delivers to an Apple Push Notification service device token and is only offered when an administrator has configured it." So this is a configuration/ops task, not an API change.
• A third-party iOS client (native DevPlace app) is blocked on this.

Environment

Not provided.

Reported by Lensflare via DevPlace.