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.go

@@ -19,6 +19,7 @@ const (
 	KindRuntimeStartConfigInvalid = "runtime.start_config_invalid"
 	KindGameTurnReady = "game.turn.ready"
 	KindGamePaused = "game.paused"
+	KindDiplomailReceived = "diplomail.message.received"
 )
 
 // CatalogEntry describes the per-kind delivery policy: which channels
@@ -103,6 +104,9 @@ var catalog = map[string]CatalogEntry{
 	KindGamePaused: {
 		Channels: []string{ChannelPush},
 	},
+	KindDiplomailReceived: {
+		Channels: []string{ChannelPush},
+	},
 }
 
 // LookupCatalog returns the per-kind policy and a boolean reporting
@@ -133,5 +137,6 @@ func SupportedKinds() []string {
 		KindRuntimeStartConfigInvalid,
 		KindGameTurnReady,
 		KindGamePaused,
+		KindDiplomailReceived,
 	}
 }