inbound-antispam-panel: the filter signs what it judged

Closes what the security review left as an accepted risk: with the
spam filter down - it is attached fail-open - nothing removes verdict
headers a sender wrote himself, and the inbound journal recorded his
verdict as the filter's. The owner asked whether rspamd could add a
header that proves the others are ours (2026-10-12) and agreed to a
signature: a fixed mark could be written by a sender as easily as the
verdict.

  - inbound.FilterKey: a key of 32 random bytes, 64 hex characters, in
    /data/antispam/filter.key, made on the first start with the relay
    on. 0644 like the list files beside it: the filter is another user
    in another container.
  - deploy/antispam/rspamd/milter_headers.conf: a custom routine reads
    the key through the directory the sidecar already mounts and adds
    X-SelfPost-Filter, the HMAC-SHA256 of the message's queue id, hex;
    a header of that name from the sender is removed.
  - internal/milter/inbound.go: the verdict headers are kept as they
    are read, and at end-of-message - when the queue id is known - they
    count only if the signature fits (FilterSignature, hmac.Equal). No
    key, no queue id, no signature or a wrong one: the row is that of
    mail nobody judged (accept, selfpost, no detail).

The signature goes on to the upstream with the mail and is of no use
there: it fits one queue id.

Tried in a throwaway rspamd 3.11 container on the test server before
the Go side was written: the routine loads, signs
(2aa1ca6d... for key 0123...cdef and queue id 4A1B2C3D5E, the value
python's hmac gives and TestFilterSignatureIsWhatTheFilterWrites
pins), removes a sender's header of the same name, and adds nothing
without a queue id.

Tests: signed, unsigned, made-up, another message's, another key's,
the sender's followed by the real one, the next message on the
connection, no queue id; the key made once, read after, replaced when
damaged, left alone by the lists. e2e: filter.key is there, 644, 65
bytes.

docs/security.md: the accepted risk is replaced by a narrower one (the
key is readable on the host); Mail path describes the signature.

Checked on the test server, uncommitted tree: go vet ./... ok,
go test ./... ok (22 packages), make e2e ok.

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

docs/architecture.md

@@ -88,7 +88,14 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   `inbound_spam_log` row per recipient at an inbound domain - a DMARC report
   address is not one - and never refuses anything
   ([internal/milter/inbound.go](../internal/milter/inbound.go)). The headers
-  are read only while a filter is attached; refused mail never reaches it.
+  are read only while a filter is attached, and only next to the filter's
+  signature: a third header, `X-SelfPost-Filter`, the HMAC-SHA256 of the
+  message's queue id under the key in `/data/antispam/filter.key`
+  (`inbound.FilterKey` makes it on first start; rspamd reads it through the
+  mounted directory and signs in a custom routine of `milter_headers.conf`).
+  With the filter down - it is attached fail-open - a sender's own verdict
+  headers carry no signature and the row is that of mail nobody judged.
+  Refused mail never reaches this milter.
   Refusals come from `mail.log`: this listener logs as `postfix/inbound`
   (`syslog_name`), and the log tailer turns a `reject:` line of
   `postfix/inbound/smtpd` (Postfix's own policy) and a `milter-reject` /