panel-dkim-signing stage 2: OpenDKIM removed

The panel's signer is the only one. Gone: the opendkim package and its
configuration, the syslog-relay and opendkim programs, /dev/log and
/run/opendkim, the reload by USR1 through supervisord
(internal/supervisor/supervisor.go), internal/syslogrelay, the port of
OpenDKIM's From: parser (internal/milter/mailparse.go and its probe,
harness and test/upstream scripts), OPENDKIM_SOCKET and OPENDKIM_DIR.

Keys: internal/domain/keys.go (Keys, NewKeys, DKIM_DIR=/data/dkim) keeps
the keys and writes a selector table, /data/dkim/selectors ("<name>
<selector>"), which the signer reads in place of the KeyTable - it reads
no database, as the sender check reads none. The table is written before
the signer serves (EnsureServerSigned moved there from the notifier).
Keys and table 0600, directories 0700, the panel's. The entrypoint copies
/data/opendkim/keys once, as the panel user, into /data/dkim.new renamed
when whole, and stops the start if the copy fails; /data/opendkim is left
for a rollback to 2.7.x.

Sender check: one RFC 5322 address as a mail program reads it, held
against the bindings; the 256-byte refusal and the texts naming OpenDKIM
go. Allowed() folded the domain through strings.ToLower, which makes the
Kelvin sign a "k" - the comparison with OpenDKIM's reading had hidden it;
it folds ASCII only now.

Health: two dkim-signer rows (applications, notifications) in Milter
sockets, opendkim out of the liveness set; the card no longer takes any
row with a note for the spam filter. Texts as in the design commit.

KILL stays in deploy/docker-compose.yml: measured on the test server,
root without it cannot signal a process of another uid - supervisord
stopping the panel, Postfix's master its children. The plan's "only use
was the reload" was wrong; the comment now gives the real uses.

e2e: one signature, the panel's, verified by dkimpy; the /data/dkim modes;
the OpenDKIM log and process checks, testNeverSignedForAnother and the
cited opendkim, libopendkim11 and libmilter1.0.1 go.

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

internal/domain/dkim.go

@@ -1,8 +1,8 @@
 // Package domain owns SelfPost's sending-domain model: per-domain DKIM key
-// generation, the OpenDKIM KeyTable/SigningTable that drive signing, and the
-// orchestration that keeps the SQLite registry, the on-disk keys and OpenDKIM
-// in agreement (architecture.md § OpenDKIM). Key material lives under /data so
-// it survives container restarts (architecture.md § OpenDKIM).
+// generation, the selector table the panel's signer reads, and the
+// orchestration that keeps the SQLite registry and the on-disk keys in
+// agreement (architecture.md § DKIM signing). Key material lives under /data
+// so it survives container restarts.
 package domain
 
 import (
@@ -32,23 +32,15 @@ func generateDKIMKey() (*rsa.PrivateKey, error) {
 }
 
 // writePrivateKeyPEM writes key to path as a PKCS#1 "RSA PRIVATE KEY" PEM,
-// atomically and group-readable (0640). The file is owned by the panel user and
-// read by OpenDKIM through the shared `selfpost` group (see build/opendkim.conf
-// and entrypoint.sh); the parent directory carries setgid so the group is
-// inherited. The write is atomic (temp file + rename) so OpenDKIM never observes
-// a half-written key.
-//
-// OpenDKIM 2.11.0~beta2, libopendkim/dkim.c, dkim_privkey_load(): a key that
-// starts as PEM goes to OpenSSL's PEM_read_bio_PrivateKey(), which takes this
-// PKCS#1 form; opendkim/opendkim.c, dkimf_add_signrequest(): a key file
-// dkimf_securefile() finds insecure is only logged while RequireSafeKeys is no
-// (build/opendkim.conf).
+// atomically and readable by the panel user alone (0600): the panel's signer
+// is the only reader (internal/milter, readKey()). The write is atomic (temp
+// file + rename) so the signer never reads a half-written key.
 func writePrivateKeyPEM(path string, key *rsa.PrivateKey) error {
 	block := &pem.Block{
 		Type:  "RSA PRIVATE KEY",
 		Bytes: x509.MarshalPKCS1PrivateKey(key),
 	}
-	return writeFileAtomic(path, pem.EncodeToMemory(block), 0o640)
+	return writeFileAtomic(path, pem.EncodeToMemory(block), 0o600)
 }
 
 // loadPrivateKeyPEM reads and parses a PKCS#1 RSA private key written by
@@ -79,7 +71,7 @@ type DKIMRecord struct {
 }
 
 // dkimRecord builds the published DKIM DNS record for a public key. The value
-// mirrors what opendkim-genkey emits: v=DKIM1, sha256, RSA, and the public key
+// is the one opendkim-genkey emitted: v=DKIM1, sha256, RSA, and the public key
 // as base64-encoded SubjectPublicKeyInfo (PKIX) DER.
 func dkimRecord(selector, domainName string, pub *rsa.PublicKey) (DKIMRecord, error) {
 	der, err := x509.MarshalPKIXPublicKey(pub)
@@ -95,7 +87,7 @@ func dkimRecord(selector, domainName string, pub *rsa.PublicKey) (DKIMRecord, er
 
 // writeFileAtomic writes data to path via a temp file in the same directory
 // followed by a rename, so readers only ever see the complete old or new file.
-// It is the single safe-write primitive for DKIM keys and OpenDKIM tables
+// It is the single safe-write primitive for DKIM keys and the selector table
 // (security.md).
 func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
 	return atomicfile.Write(path, data, perm)