panel-redesign: no report mailbox for the server as a whole

Follows the design commit before it.

  - internal/dmarc: the ingest allow-list is the report address of each
    domain that is on this host and nothing else;
    DefaultHostedReportAddress and DefaultHostedSuggestion are removed.
    Since 1.7.0 the general mailbox dmarc-reports@<hostname> was open
    while no panel-wide default existed; the three-choice scheme had
    left it open always.
  - The DMARC hub's Ingest box shows the form of a hosted address,
    dmarc-reports+<domain>@<hostname>.
  - guide.md: what port 25 accepts.

Evidence: docs/assets/panel-redesign/evidence/dmarc.png re-shot (the
address form wraps after the plus sign in its column at 1440 px).

  go test -count=1 ./internal/dmarc/... ./internal/web/... ./cmd/panel: ok

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

internal/dmarc/service_test.go

@@ -7,8 +7,8 @@ import (
 	"github.com/mixeme/selfpost/internal/store"
 )
 
-// The ingest allow-list is the generic mailbox plus the report address of each
-// domain that points at this host: a new domain (no reports) and a domain
+// The ingest allow-list is the report address of each domain that points at
+// this host, and nothing for the server as a whole: a new domain (no reports) and a domain
 // reporting to another host open nothing of their own.
 func TestAllowedRecipientsFollowTheDomainAddresses(t *testing.T) {
 	st, err := store.Open(t.TempDir() + "/test.db")
@@ -36,7 +36,7 @@ func TestAllowedRecipientsFollowTheDomainAddresses(t *testing.T) {
 	if err != nil {
 		t.Fatalf("AllowedRecipients: %v", err)
 	}
-	want := []string{"dmarc-reports+hosted.example@mail.example.com", "dmarc-reports@mail.example.com", "ops@mail.example.com"}
+	want := []string{"dmarc-reports+hosted.example@mail.example.com", "ops@mail.example.com"}
 	if !reflect.DeepEqual(got, want) {
 		t.Errorf("allowed recipients = %v, want %v", got, want)
 	}