release: 1.4.0: Optional inbound relay (backup-MX) on port 25

Optional inbound relay (backup-MX / forwarder) on port 25, off by default. Mail is accepted only for domains configured in the panel and forwarded to an upstream; there are still no mailboxes. A dedicated security review of the inbound path is still open on the plan (Fable).
Added:
  - `INBOUND_RELAY_ENABLE` (default `false`): when true, Postfix listens on
  - Panel **Inbound** pages (global administrator): domains, upstream host/port
  - Optional `INBOUND_ANTISPAM_MILTER` on the inbound listener only, plus
  - Inbound rate limit (`INBOUND_RATE_LIMIT_MESSAGES_PER_IP`, default 20) and
Changed:
  - Compose publishes host port 25 even when the flag is off (nothing listens
  - Full-backup restore Resync also rebuilds inbound maps when the flag is on.

Full detail: CHANGELOG.md (1.4.0).

internal/web/handlers/handlers_monitor_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/mixeme/selfpost/internal/inbound"
 	"github.com/mixeme/selfpost/internal/postfix"
 	"github.com/mixeme/selfpost/internal/store"
 	"github.com/mixeme/selfpost/internal/web/auth"
@@ -347,7 +348,12 @@ func serverWithTwoDomains(t *testing.T) (*Handlers, map[string]store.Domain) {
 		domains[d.name] = dom
 	}
 
-	return &Handlers{store: st, view: mustView(t), cfg: Config{Version: "test"}}, domains
+	return &Handlers{
+		store:   st,
+		inbound: inbound.NewService(st, &recordingMaps{}),
+		view:    mustView(t),
+		cfg:     Config{Version: "test", InboundEnabled: true},
+	}, domains
 }
 
 var globalPrincipal = auth.Principal{ID: 1, Username: "admin", Role: auth.RoleGlobal}