Track an App interaction event

Record a user interaction event for a public App — no authentication required.

Interaction events are written to the app_interactions collection for analytics and relevance tuning. Write errors are swallowed server-side so that a transient storage failure never surfaces as an error to the caller.

Example:

{
  "event_type": "click",
  "document_id": "doc_abc123",
  "position": 2,
  "query": "red sneakers",
  "session_id": "sess_xyz"
}