Onboarding: track first SDK evaluation received (Core)

[talissoncosta]

Expose an API that can be polled to determine if an environment has completed onboarding.

For SaaS, expose an API that allows setting onboarding state for an environment.

Acceptance criteria

• The environment document Dynamo schema is updated with onboarding_pending: bool | null field.
• GET /api/v1/environments/{environment_api_key}/onboarding-status/ returns first_evaluated_at: datetime | null, first_evaluated_sdk_label: str | null.
• PUT /api/v1/environments/{environment_api_key}/onboarding-status/ persists first_evaluated_at and first_evaluated_sdk_label. It's authenticated via environment API key, or equivalent auth sufficient to be called from inside an Edge API lambda.
• No new queries fired during SDK API operation for onboarded environments (including pre-existing environments).