Track one product event

Records a single product event for the app's user. Event meaning is versioned by event_code: if the meaning of an event ever changes (what it counts, or when the host app emits it), the host must emit a new event_code. Existing codes are never redefined. Events sent with only an anonymous_id attach to an anonymous user, which is listed on the dashboard but not evaluated until an identify call promotes it. For an app that ingests asynchronously, the request is validated, durably accepted into the ingest buffer and answered 202 with {"event":{"mapping_status":"pending"},"processing":"queued"}: no event, user or job yet, since the event is applied shortly after, in the order the app's requests were accepted. Failures that need the database (a derived event code, a contact-rule violation) are then not answered to the caller; they are dead-lettered (see the dashboard ingest dead-letters endpoint). A retried request with the same context.$message_id is still deduplicated. Events from known crawlers and bots (the request's User-Agent, or the event's context.user_agent) are answered 202 with the queued shape but dropped without being stored (INGEST_DROP_BOTS).