Apply a delta mutation to a contact.

Delta mutation for one existing contact. Accepts any combination of addTags, removeTags, addLists, removeLists, and updateFields; omitted operations leave the corresponding facet unchanged. Status changes are handled by PATCH /api/contact/:contactId, not this endpoint. Operations are atomic — all-or-nothing in a single save. Idempotent: adding a tag already present, removing a tag not present, or setting a field to its current value is a no-op and is rejected with 400 only when every operation in the request resolves to no change. __v is optional: when supplied, optimistic-concurrency control is enforced (stale version → 400 ConcurrencyError); when omitted, the request runs last-write-wins.