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/jet/backend/model/diplomail_translations.go

@@ -0,0 +1,23 @@
+//
+// Code generated by go-jet DO NOT EDIT.
+//
+// WARNING: Changes to this file may cause incorrect behavior
+// and will be lost if the code is regenerated
+//
+
+package model
+
+import (
+	"github.com/google/uuid"
+	"time"
+)
+
+type DiplomailTranslations struct {
+	TranslationID     uuid.UUID `sql:"primary_key"`
+	MessageID         uuid.UUID
+	TargetLang        string
+	TranslatedSubject string
+	TranslatedBody    string
+	Translator        string
+	TranslatedAt      time.Time
+}