fe_comment: recipients are the FE users of the feedback group

- 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

packages/fe_comment/ext_conf_template.txt

@@ -19,7 +19,7 @@ jsonOutput = 1
 # cat=access//10; type=string; label=FE user group (uid): Only logged-in frontend users in this group see and use the widget. 0 = any logged-in user.
 feUserGroup = 0
 
-# cat=mail//50; type=string; label=Selectable recipients: who the feedback can be sent to, as "Label <mail@example.com>" entries separated by semicolons. Empty = only the Mail to address above.
+# cat=mail//50; type=string; label=Selectable recipients (UNUSED since 2026-10-09): recipients are now the frontend users of the FE user group above that have an e-mail address. Kept only so existing configurations stay valid.
 recipients =
 
 # cat=basic//20; type=string; label=Storage pid: page/folder uid the feedback records are stored on (backend visibility). 0 = root.