diplomail (Stage A): add in-game personal mail subsystem

Phase 28 of ui/PLAN.md needs a persistent player-to-player mail
channel; the existing `mail` package is a transactional email
outbox and the `notification` catalog is one-way platform events.
Stage A lands the schema (diplomail_messages / _recipients /
_translations), a single-recipient personal send/read/delete
service path, a `diplomail.message.received` push kind plumbed
through the notification pipeline, and an unread-counts endpoint
that drives the lobby badge. Admin / system mail, lifecycle hooks,
paid-tier broadcast, multi-game broadcast, bulk purge and language
detection / translation cache come in stages B–D.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/postgres/migrations_test.go

@@ -68,6 +68,10 @@ var expectedBackendTables = []string{
 	"notification_malformed_intents",
 	"notification_routes",
 	"notifications",
+	// Diplomail domain.
+	"diplomail_messages",
+	"diplomail_recipients",
+	"diplomail_translations",
 	// Geo domain.
 	"user_country_counters",
 }