Get unread notification count (badge poll)

Lightweight endpoint for the badge poll — returns only the running unread count, never the full notification list.

curl -X GET "https://example.com/v1/me/notifications/unread-count"

{
  "unread_count": 3
}

{
  "error": "unauthenticated",
  "message": "missing or invalid bearer token",
  "request_id": "req_01abc123"
}