Add PUT endpoint for Social Connections

Integrations can partially update Social Connection attributes via
PUT /api/v1/social_connections/:id. Writable fields include kinship metadata,
emotional relationship statuses, and external identifier. Omitted attributes are
unchanged; explicit null clears nullable fields. Relationship keys in the PUT
body are rejected by JSON schema validation.

POST for a pair that already has a social connection returns 409 Conflict
without changing the existing row. Use PUT on that connection's id to update it.