Upserts the user's per-category mute flags and returns the recomputed effective preferences. Also served on PATCH. Requires a token with scope_priv on the notificationpreference object, with the same self-or-administrator confinement as the read. Addressing the user=* domain-default row is restricted to administrators. The body is a map of category to a boolean enabled flag, accepted either wrapped in a preferences object or as a flat map. Category keys longer than 32 characters, and empty keys, are skipped. The write is an upsert keyed on domain, user, and category, so sending a subset updates only those categories.