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/store/domains_test.go

@@ -0,0 +1,117 @@
+package store
+
+import (
+	"errors"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func openTestStore(t *testing.T) *Store {
+	t.Helper()
+	st, err := Open(filepath.Join(t.TempDir(), "test.db"))
+	if err != nil {
+		t.Fatalf("open store: %v", err)
+	}
+	t.Cleanup(func() { st.Close() })
+	return st
+}
+
+func TestAddAndListDomains(t *testing.T) {
+	st := openTestStore(t)
+
+	d, err := st.AddDomain("example.com", "selfpost")
+	if err != nil {
+		t.Fatalf("AddDomain: %v", err)
+	}
+	if d.ID == 0 || d.Name != "example.com" || d.DKIMSelector != "selfpost" {
+		t.Fatalf("unexpected domain: %+v", d)
+	}
+
+	list, err := st.ListDomains()
+	if err != nil {
+		t.Fatalf("ListDomains: %v", err)
+	}
+	if len(list) != 1 || list[0].Name != "example.com" || list[0].AppCount != 0 {
+		t.Fatalf("unexpected list: %+v", list)
+	}
+}
+
+func TestAddDomainDuplicate(t *testing.T) {
+	st := openTestStore(t)
+	if _, err := st.AddDomain("example.com", "selfpost"); err != nil {
+		t.Fatal(err)
+	}
+	_, err := st.AddDomain("example.com", "other")
+	if !errors.Is(err, ErrDomainExists) {
+		t.Fatalf("duplicate AddDomain error = %v, want ErrDomainExists", err)
+	}
+}
+
+func TestGetDomainNotFound(t *testing.T) {
+	st := openTestStore(t)
+	if _, err := st.GetDomain(999); !errors.Is(err, ErrDomainNotFound) {
+		t.Fatalf("GetDomain(missing) = %v, want ErrDomainNotFound", err)
+	}
+}
+
+func TestDeleteDomainCascadesApplications(t *testing.T) {
+	st := openTestStore(t)
+	d, err := st.AddDomain("example.com", "selfpost")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Insert an application + address directly (the AddApplication API lands in
+	// Phase 4); this verifies the ON DELETE CASCADE wiring now.
+	now := time.Now().UTC().Format(time.RFC3339)
+	res, err := st.db.Exec(
+		"INSERT INTO applications (domain_id, login, address_mode, created_at) VALUES (?, ?, 'wildcard', ?)",
+		d.ID, "app1", now)
+	if err != nil {
+		t.Fatal(err)
+	}
+	appID, _ := res.LastInsertId()
+	if _, err := st.db.Exec(
+		"INSERT INTO application_addresses (application_id, address) VALUES (?, ?)",
+		appID, "noreply@example.com"); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := st.GetDomain(d.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.AppCount != 1 {
+		t.Fatalf("AppCount = %d, want 1", got.AppCount)
+	}
+
+	if err := st.DeleteDomain(d.ID); err != nil {
+		t.Fatalf("DeleteDomain: %v", err)
+	}
+
+	assertCount(t, st, "applications", 0)
+	assertCount(t, st, "application_addresses", 0)
+	if _, err := st.GetDomain(d.ID); !errors.Is(err, ErrDomainNotFound) {
+		t.Errorf("domain still present after delete: %v", err)
+	}
+}
+
+func TestDeleteDomainNotFound(t *testing.T) {
+	st := openTestStore(t)
+	if err := st.DeleteDomain(123); !errors.Is(err, ErrDomainNotFound) {
+		t.Fatalf("DeleteDomain(missing) = %v, want ErrDomainNotFound", err)
+	}
+}
+
+func assertCount(t *testing.T, st *Store, table string, want int) {
+	t.Helper()
+	var n int
+	// table is a trusted literal from the test, not user input.
+	if err := st.db.QueryRow("SELECT COUNT(*) FROM " + table).Scan(&n); err != nil {
+		t.Fatalf("count %s: %v", table, err)
+	}
+	if n != want {
+		t.Errorf("%s count = %d, want %d", table, n, want)
+	}
+}