Messaging - per-provider APNs/FCM passthrough (Live Activity, mutable-content, custom headers) + ActivityKit push tokens

### 🔖 Enhancement description Appwrite Messaging's push API can only send standard alert/data notifications. It has no way to: 1. **Set custom APNs headers / payload keys** — specifically `apns-push-type`, the `apns-topic` suffix, and payload keys such as `mutable-content`. 2. **Register or target ActivityKit push tokens** — the *push-to-start* token and per-activity *update* token iOS returns for Live Activities. These are distinct APNs tokens, not the FCM registration / normal device token. More broadly, the `Push` message is a **closed field set**, so *any* Apple/Google field it doesn't enumerate is unreachable — Live Activities and NSE rich push are just the two that block real apps today. This request is to add an optional **per-provider passthrough** (`apns` / `fcm`) on `createPush`, deep-merged into the payload/headers the adapters already build, plus a push-target type for ActivityKit tokens. This mirrors FCM's own HTTP v1 API and is purely additive. I (with my AI agent) verified against current source (`appwrite/appwrite@main`, and `utopia-php/messaging` **v2.0.1** which it pins — line-level receipts below). Neither the pinned version nor `utopia-php/messaging@main` supports any of this. ### 🎤 Pitch ### Why it's impossible today — line-level evidence **1. The public `createPush` surface is a closed field set** (`app/controllers/api/messaging.php`): ``` messageId, title, body, topics, users, targets, data, action, image, icon, sound, color, tag, badge, draft, scheduledAt, contentAvailable, critical, priority ``` No `pushType`, `apnsHeaders`, `mutableContent`, `liveActivity` / `contentState`. `image` must be an Appwrite Storage `bucket:file` (`CompoundUID`), so it can't even point at an arbitrary trigger payload. **2. The worker construction is a hard ceiling** (`src/Appwrite/Platform/Workers/Messaging.php`): ```php return new Push($to, $title, $body, $data, $action, $sound, $image, $icon, $color, $tag, $badge, $contentAvailable, $critical, …