Add optional inbound relay (backup-MX) behind INBOUND_RELAY_ENABLE.

Port 25 accepts only configured domains and listed recipients, then forwards to an upstream; the outbound path is unchanged when the flag is off.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/store/store.go

@@ -1,8 +1,8 @@
 // Package store owns the SelfPost SQLite database: the single file under /data
 // that persists panel users (global administrators and domain-admins), sending
-// domains and applications, the send log and rate-limit settings
-// (architecture.md § Persistence). It exposes typed queries so the rest of the
-// panel never builds SQL by hand.
+// domains and applications, optional inbound-relay domains, the send log and
+// rate-limit settings (architecture.md § Persistence). It exposes typed queries
+// so the rest of the panel never builds SQL by hand.
 package store
 
 import (