Update a partial subset of group settings

Accepts any subset of the sections returned by GET /groups/{group_id}/settings.
Each section is independently optional — sending only meeting updates only
meeting fields. Within a section, only properties that are explicitly set are
persisted. Returns the full updated settings shape on success.

Concurrency model: last-write-wins (no optimistic-locking version field).

Authorization: caller must have canEditGroup. Group Leaders are blocked
(403) from writing the privileges, organization, or attendance_grouping_ids
sections — those require admin / department-admin / master-admin rights.

Gated by the edit_group_settings feature flag — returns 405 when the flag is off.