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/web/web.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"time"
 
+	"codeberg.org/mix/selfpost/internal/domain"
 	"codeberg.org/mix/selfpost/internal/store"
 )
 
@@ -30,6 +31,7 @@ type Config struct {
 // Server is the panel HTTP application.
 type Server struct {
 	store    *store.Store
+	domains  *domain.Service
 	cfg      Config
 	tmpl     *templates
 	sessions *sessionStore
@@ -40,14 +42,16 @@ type Server struct {
 }
 
 // New builds the panel server. setupTokenPath is where the current setup token
-// is mirrored on disk (spec 7.6.1).
-func New(st *store.Store, cfg Config, setupTokenPath string) (*Server, error) {
+// is mirrored on disk (spec 7.6.1); domains is the sending-domain service that
+// owns DKIM keys and the OpenDKIM tables (spec 6).
+func New(st *store.Store, domains *domain.Service, cfg Config, setupTokenPath string) (*Server, error) {
 	tmpl, err := loadTemplates()
 	if err != nil {
 		return nil, err
 	}
 	s := &Server{
 		store:    st,
+		domains:  domains,
 		cfg:      cfg,
 		tmpl:     tmpl,
 		sessions: newSessionStore(),
@@ -85,8 +89,16 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("/login", s.handleLogin)
 	mux.HandleFunc("/logout", s.handleLogout)
 
-	// Authenticated panel.
-	mux.Handle("/", s.requireAuth(http.HandlerFunc(s.handleDashboard)))
+	// Authenticated panel. Everything not matched by a more specific pattern
+	// above falls through to this sub-mux, wrapped once in the auth middleware.
+	authed := http.NewServeMux()
+	authed.HandleFunc("GET /{$}", s.handleDashboard)
+	authed.HandleFunc("POST /domains", s.handleAddDomain)
+	authed.HandleFunc("GET /domains/{id}", s.handleDomainDetail)
+	authed.HandleFunc("GET /domains/{id}/delete", s.handleDeleteConfirm)
+	authed.HandleFunc("POST /domains/{id}/delete", s.handleDeleteDomain)
+	authed.HandleFunc("POST /reload", s.handleReload)
+	mux.Handle("/", s.requireAuth(authed))
 
 	return mux
 }