spp_api_v2: GroupService._create_members logs identifier values (PII in logs) #566

Problem

GroupService._create_members() logs "Individual not found: %s|%s", system, value
(spp_api_v2/services/group_service.py, ~416), writing the identifier value a client sent (e.g. a national ID) to the
server log. docs/principles/error-handling.md / the repo checklist require no PII in log messages.

PR #555 removed similar identifier-echoing warnings from the search filters.

Proposal

Log without the value (e.g. the identifier type only), and check the other spp_api_v2 log lines that format
namespace_uri|value for the same issue.