[bruel-gabrielsson]

Lets a site record its users' events with the SDK and get results from those stored histories, instead of sending the history on every call.

• client.events.track(user_id, event, catalog_id=, timestamp=) posts to /v1/events and returns the event id. An Order is sent as the API stores it, with a cart_id (generated if not given) and unit_selling_price 0. A naive timestamp is refused.
• client.events.list(user_id=, limit=, types=) reads GET /v1/events/own, which returns only the calling account's events (Unbox-AI/unbox-api#590).
• complete(user_id=...) and sections(user_id=...) send user_id with no history. The only event allowed alongside it is the current Search, which is recorded as the user's event by default (register_event=False turns that off). Without user_id, nothing is recorded unless register_event=True, as before.
• history now defaults to empty, so client.complete(user_id="u1") works.
• The async client gets the same methods.

The server side is on deploy/behaviorgpt in unbox-api and is off by default (EVENTS_SCOPED_TO_ACCOUNT, EVENTS_PUBLISHABLE_KEYS, features.events). These calls return errors until it is turned on for the BehaviorGPT deployment, so hold the release until then.

Tests: 19 new in tests/test_events.py; all 192 pass; ruff clean.