- The selectable recipients are no longer a hand-maintained setting but
every active frontend user of the feedback group (feUserGroup) with a
valid e-mail address; the id is the fe_users uid, so a stored selection
survives users joining or leaving. The `recipients` setting is marked
unused
- Each recipient in the state payload carries label, email and selected
(the stored choice), for a checkbox list with "select all" above the
submit button
- submit accepts the checked ids directly ({"recipients": [uids]}),
validates them against the group, stores and uses them, and reports
the addresses used as sentTo; no selection falls back to mailTo
- migrations/check_fecomment_user.php: all summaries and items of one
user incl. deleted ones, for diagnosis
