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/events_test.go

@@ -25,9 +25,15 @@ import (
 // payload is `{game_id, turn, reason}` consumed by the same in-game
 // shell layout, so there is no value in dragging a FB schema in for
 // one consumer.
+//
+// `diplomail.message.received` (Stage A) carries the message metadata
+// plus an unread-count snapshot. Stage A intentionally ships the
+// payload as JSON so the diplomail UI can iterate on the contract
+// without a FB schema dance; a later stage can promote it.
 var jsonFriendlyKinds = map[string]bool{
-	KindGameTurnReady: true,
-	KindGamePaused:    true,
+	KindGameTurnReady:     true,
+	KindGamePaused:        true,
+	KindDiplomailReceived: true,
 }
 
 // TestBuildClientPushEventCoversCatalog asserts that every catalog kind
@@ -88,6 +94,17 @@ func TestBuildClientPushEventCoversCatalog(t *testing.T) {
 			"turn":    int32(7),
 			"reason":  "generation_failed",
 		}},
+		{"diplomail message received", KindDiplomailReceived, map[string]any{
+			"message_id":   gameID.String(),
+			"game_id":      gameID.String(),
+			"kind":         "personal",
+			"sender_kind":  "player",
+			"subject":      "Trade deal",
+			"preview":      "Care to talk gas mining?",
+			"preview_lang": "en",
+			"unread_total": 3,
+			"unread_game":  1,
+		}},
 	}
 
 	seenKinds := map[string]bool{}