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/web/validate/validate.go

@@ -52,15 +52,15 @@ func isASCIILetterOrDigit(r rune) bool {
 const maxDomainLen = 253 // RFC 1035 limit on a fully-qualified name
 
 // NormalizeDomain lower-cases and trims a domain name. Domain names are
-// case-insensitive, and the generated OpenDKIM tables/keys use the canonical
+// case-insensitive, and the DKIM selector table and keys use the canonical
 // lower-case form, so we normalise before both validation and storage.
 func NormalizeDomain(name string) string {
 	return strings.ToLower(strings.TrimSpace(name))
 }
 
 // Domain enforces a strict server-side whitelist for sending-domain names
-// (security.md). The result is safe to write verbatim into the OpenDKIM
-// KeyTable/SigningTable and to use as a filesystem path segment: only
+// (security.md). The result is safe to write verbatim into the DKIM
+// selector table and to use as a filesystem path segment: only
 // lower-case letters, digits, '.' and '-' are allowed, in valid DNS label
 // shape. Input must already be normalised with NormalizeDomain.
 func Domain(name string) error {
@@ -85,7 +85,7 @@ func Domain(name string) error {
 // Selector checks a DKIM selector: one or more DNS labels as Domain allows
 // them, at most 63 characters in all. The panel's own selector is fixed in
 // its configuration; this is for one read from a domain export file, which
-// goes into OpenDKIM's tables, a key file's name and every signature's s=
+// goes into the selector table, a key file's name and every signature's s=
 // (review of audit stage 2).
 func Selector(s string) error {
 	if s == "" {