Api User Org Settings Retrieve

Read + partial-update serializer for a user's per-org settings (#2469).

Reads and writes the pins column of the UserOrgSetting row. pins is the ordered list of {kind, ref, title, href} dicts, checked element-by- element via PinItem (the dashboard-widgets idiom: a Pydantic item model + the shared indexed-error parser), with the PIN_LIMIT cap layered on in validate_pins. The kind allowlist is enforced by the Pydantic model itself (kind is a PinKind literal), so there is no separate allowlist check here. Doubles as the read serializer: UserOrgSettingSerializer(instance).data returns {pins} off the model -- an unsaved default instance serializes as the empty {pins: []} baseline a never-written (user, org) reports.