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/notification/catalog_test.go

@@ -41,6 +41,7 @@ func TestCatalogChannels(t *testing.T) {
 		KindRuntimeStartConfigInvalid:   {ChannelEmail},
 		KindGameTurnReady:               {ChannelPush},
 		KindGamePaused:                  {ChannelPush},
+		KindDiplomailReceived:           {ChannelPush},
 	}
 	for kind, want := range expect {
 		entry, ok := LookupCatalog(kind)