inbound-antispam-panel: the filter reads its signing key for every message

The routine kept the key after its first read. SelfPost makes a new key
when the file is lost or damaged, or a backup brings back another, and
rspamd would then have gone on signing with the old one - every verdict
unrecorded - until someone restarted the sidecar. The file is one short
line; reading it per message costs nothing worth keeping a copy for.

The guide says what never needs doing: nothing of rspamd's is cleared
after a change of the lists or the key; a restart is for a change of
its own configuration only.

Prompted by the owner's question whether rspamd's cache needs clearing
(2026-10-12).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/antispam/rspamd/milter_headers.conf

@@ -21,15 +21,17 @@ use = ["x-spamd-result", "x-rspamd-action", "selfpost-signature"];
 
 custom {
   selfpost-signature = <<EOD
-local key
 return function(task, common_meta)
   local remove = { ['X-SelfPost-Filter'] = 0 }
-  if not key then
-    local f = io.open('/etc/rspamd/selfpost/filter.key', 'r')
-    if f then
-      key = f:read('*l')
-      f:close()
-    end
+  -- The key is read for every message and kept nowhere: SelfPost may make a
+  -- new one (the file was lost, a backup was restored), and a key remembered
+  -- here would go on signing what SelfPost no longer accepts until someone
+  -- restarted this container. The file is one short line.
+  local key
+  local f = io.open('/etc/rspamd/selfpost/filter.key', 'r')
+  if f then
+    key = f:read('*l')
+    f:close()
   end
   local qid = task:get_queue_id()
   if not key or key == '' or not qid or qid == '' then