feat: implement JMAP push notification handling and subscription management

lib/push/validation.ts

@@ -0,0 +1,12 @@
+export function isValidSubscriptionId(id: unknown): id is string {
+  return typeof id === 'string' && /^[A-Za-z0-9_-]{8,128}$/.test(id);
+}
+
+export function isValidExpoPushToken(token: unknown): token is string {
+  if (typeof token !== 'string') return false;
+  // Expo managed tokens, or raw FCM/APNs tokens for bare workflows.
+  return (
+    /^ExponentPushToken\[[A-Za-z0-9_-]+\]$/.test(token) ||
+    /^[A-Za-z0-9:_-]{40,250}$/.test(token)
+  );
+}