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/server/router.go

@@ -68,6 +68,7 @@ type RouterDependencies struct {
 	UserLobbyMy *UserLobbyMyHandlers
 	UserLobbyRaceNames *UserLobbyRaceNamesHandlers
 	UserGames *UserGamesHandlers
+	UserMail *UserMailHandlers
 	UserSessions *UserSessionsHandlers
 	AdminAdminAccounts *AdminAdminAccountsHandlers
 	AdminUsers *AdminUsersHandlers
@@ -163,6 +164,9 @@ func withDefaultHandlers(deps RouterDependencies) RouterDependencies {
 	if deps.UserGames == nil {
 		deps.UserGames = NewUserGamesHandlers(nil, nil, deps.Logger)
 	}
+	if deps.UserMail == nil {
+		deps.UserMail = NewUserMailHandlers(nil, deps.Logger)
+	}
 	if deps.UserSessions == nil {
 		deps.UserSessions = NewUserSessionsHandlers(nil, deps.Logger)
 	}
@@ -255,6 +259,9 @@ func registerUserRoutes(router *gin.Engine, instruments *metrics.Instruments, de
 	my.GET("/invites", deps.UserLobbyMy.Invites())
 	my.GET("/race-names", deps.UserLobbyMy.RaceNames())
 
+	lobbyMail := lobbyGroup.Group("/mail")
+	lobbyMail.GET("/unread-counts", deps.UserMail.UnreadCounts())
+
 	raceNames := lobbyGroup.Group("/race-names")
 	raceNames.POST("/register", deps.UserLobbyRaceNames.Register())
 
@@ -265,6 +272,14 @@ func registerUserRoutes(router *gin.Engine, instruments *metrics.Instruments, de
 	userGames.GET("/:game_id/reports/:turn", deps.UserGames.Report())
 	userGames.GET("/:game_id/battles/:turn/:battle_id", deps.UserGames.Battle())
 
+	userMail := userGames.Group("/:game_id/mail")
+	userMail.POST("/messages", deps.UserMail.SendPersonal())
+	userMail.GET("/messages/:message_id", deps.UserMail.Get())
+	userMail.POST("/messages/:message_id/read", deps.UserMail.MarkRead())
+	userMail.DELETE("/messages/:message_id", deps.UserMail.Delete())
+	userMail.GET("/inbox", deps.UserMail.Inbox())
+	userMail.GET("/sent", deps.UserMail.Sent())
+
 	userSessions := group.Group("/sessions")
 	userSessions.GET("", deps.UserSessions.List())
 	userSessions.POST("/revoke-all", deps.UserSessions.RevokeAll())