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>

cmd/panel/journal.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/mixeme/selfpost/internal/inbound"
 	"github.com/mixeme/selfpost/internal/milter"
 	"github.com/mixeme/selfpost/internal/store"
 )
@@ -33,8 +34,18 @@ func serveInboundJournal(ctx context.Context, cfg config, st *store.Store) error
 	if err != nil {
 		return err
 	}
+	// With a filter attached, the key it signs with. Without the key no
+	// verdict can be told from a sender's own, so none is read: the journal
+	// still records the mail, and the log says why it shows no verdicts.
+	key := ""
+	if cfg.inboundAntispamMilter != "" {
+		if key, err = inbound.FilterKey(cfg.antispamDir); err != nil {
+			log.Printf("inbound-journal-milter: no key for the filter's signature, verdicts will not be recorded: %v", err)
+			key = ""
+		}
+	}
 	log.Printf("inbound-journal-milter listening on %s", cfg.inboundJournalSocket)
-	return milter.ServeInbound(ctx, ln, st, cfg.inboundAntispamMilter != "")
+	return milter.ServeInbound(ctx, ln, st, key)
 }
 
 // listenMilter opens a milter's Unix socket where Postfix can reach it.