Phase 3: sending domains + per-domain OpenDKIM signing

Add/list/delete of sending domains with per-domain DKIM keys and the
OpenDKIM tables that drive signing (spec 6, 7.2.2-4, 7.2.10).

internal/domain:
- Pure-Go RSA-2048 keygen; PKCS#1 PEM written atomically at 0640; the
  published DNS TXT record is derived from the key on disk (single source
  of truth) rather than persisted. No os/exec for key generation.
- KeyTable/SigningTable fully regenerated from the registry on every
  add/delete (idempotent), written atomically; SigningTable via refile:
  with *@domain, KeyTable with absolute key paths. Table writer refuses
  any unsafe character as a backstop (spec 7.6.4).
- Reload without root: the unprivileged panel signals OpenDKIM through
  supervisord (`supervisorctl signal USR1 opendkim`, fixed args, no
  shell, no user input — spec 7.6.3). An existing key is reused, never
  overwritten, so re-adding a domain keeps its published DNS valid.
- Service orchestrates registry -> key -> table rebuild -> reload, with
  rollback of the row if a downstream step fails; delete cascades apps
  via the DB FK and removes the key + table entries.

Infra:
- Shared `selfpost` group bridges panel (writes keys) and opendkim
  (reads them); /data/opendkim is setgid so panel-created files inherit
  the group, keys are 0640, RequireSafeKeys is disabled by design.
- opendkim.conf moves from verify-only (Mode v) to signing (Mode s).
- entrypoint.sh normalises the DKIM tree on every start (ownership,
  setgid, perms, empty tables before opendkim starts) — self-healing
  after a restore.
- supervisord control socket opened to the `selfpost` group so the panel
  can request the reload.

web/store:
- Strict domain-name validation (whitelist [a-z0-9.-], DNS shape, >=2
  labels), lower-case normalisation (spec 7.6.2).
- Domain queries with application counts; delete relies on ON DELETE
  CASCADE. Dashboard lists domains + add form; domain page shows the
  DKIM record; a dedicated confirm page warns about the app cascade
  before deletion (spec 7.2.4); manual reload button (spec 7.2.12,
  OpenDKIM side; Postfix reload lands in Phase 5).
- Authenticated routes moved to a sub-mux using Go 1.22 method/wildcard
  patterns.

Tests: validateDomain, DKIM keygen/record roundtrip, table rendering +
injection-safety, key reuse, store cascade. Verified on the dev server:
gofmt/vet/test green, image builds, container e2e (add/delete a domain,
DKIM record shown, OpenDKIM reads panel keys and reloads, keys and
tables persist across a restart).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/domain/dkim_test.go

@@ -0,0 +1,86 @@
+package domain
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestWriteLoadPrivateKeyRoundtrip(t *testing.T) {
+	key, err := generateDKIMKey()
+	if err != nil {
+		t.Fatalf("generateDKIMKey: %v", err)
+	}
+	path := filepath.Join(t.TempDir(), "selfpost.private")
+	if err := writePrivateKeyPEM(path, key); err != nil {
+		t.Fatalf("writePrivateKeyPEM: %v", err)
+	}
+
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("stat: %v", err)
+	}
+	if perm := info.Mode().Perm(); perm != 0o640 {
+		t.Errorf("key perm = %o, want 0640", perm)
+	}
+
+	loaded, err := loadPrivateKeyPEM(path)
+	if err != nil {
+		t.Fatalf("loadPrivateKeyPEM: %v", err)
+	}
+	if loaded.N.Cmp(key.N) != 0 || loaded.E != key.E {
+		t.Error("loaded key does not match generated key")
+	}
+}
+
+func TestLoadPrivateKeyRejectsGarbage(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "bad.private")
+	if err := os.WriteFile(path, []byte("not a pem"), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := loadPrivateKeyPEM(path); err == nil {
+		t.Error("expected error for non-PEM key file")
+	}
+}
+
+func TestDKIMRecord(t *testing.T) {
+	key, err := generateDKIMKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+	rec, err := dkimRecord("selfpost", "example.com", &key.PublicKey)
+	if err != nil {
+		t.Fatalf("dkimRecord: %v", err)
+	}
+	if rec.Name != "selfpost._domainkey.example.com" {
+		t.Errorf("record name = %q", rec.Name)
+	}
+	for _, want := range []string{"v=DKIM1", "h=sha256", "k=rsa", "p="} {
+		if !strings.Contains(rec.Value, want) {
+			t.Errorf("record value %q missing %q", rec.Value, want)
+		}
+	}
+}
+
+func TestWriteFileAtomicOverwrite(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "f")
+	if err := writeFileAtomic(path, []byte("one"), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	if err := writeFileAtomic(path, []byte("two"), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "two" {
+		t.Errorf("content = %q, want %q", got, "two")
+	}
+	// No stray temp files left behind in the directory.
+	entries, _ := os.ReadDir(filepath.Dir(path))
+	if len(entries) != 1 {
+		t.Errorf("expected 1 file after atomic writes, found %d", len(entries))
+	}
+}