Link external records to people and companies

Idempotently link external platform records (e.g. Stripe customers) to Modem people and companies.

Each person / company block states the facts the platform knows about that side of the record — not an assertion of how to resolve it. Modem owns all matching.

Person — matched by platform identity first, then by exact email. A bare email matches an existing person but never creates one (it may be a shared inbox like billing@); creation requires displayName.

Company — matched by the first hit, in trust order:

1. an existing platform identity link for this block
2. the resolved person's current company
3. stated domains, then the domain derived from person.email (free-mail domains such as gmail.com carry no signal)
4. an exact, case-insensitive name match
5. otherwise created — only when name is stated

When both a person and a company resolve, their membership is recorded.

If the platform identity and a natural key (email/domain) resolve to different entities, the identity match wins and nothing is merged; the other entity is returned as conflictWith — a probable duplicate to reconcile deliberately.

Removing a stored link is not part of this endpoint; it will ship separately.