diplomail (Stage B): admin/owner sends + lifecycle hooks

Item 7 of the spec wants game-state and membership-state changes to
land as durable inbox entries the affected players can re-read after
the fact — push alone times out of the 5-minute ring buffer. Stage B
adds the admin-kind send matrix (owner-driven via /user, site-admin
driven via /admin) plus the lobby lifecycle hooks: paused / cancelled
emit a broadcast system mail to active members, kick / ban emit a
single-recipient system mail to the affected user (which they keep
read access to even after the membership row is revoked, per item 8).

Migration relaxes diplomail_messages_kind_sender_chk so an owner
sending kind=admin keeps sender_kind=player; the new
LifecyclePublisher dep on lobby.Service is wired through a thin
adapter in cmd/backend/main, mirroring how lobby's notification
publisher is plumbed today.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/lobby/lobby.go

@@ -124,6 +124,7 @@ type Deps struct {
 	Cache *Cache
 	Runtime RuntimeGateway
 	Notification NotificationPublisher
+	Diplomail DiplomailPublisher
 	Entitlement EntitlementProvider
 	Policy *Policy
 	Config config.LobbyConfig
@@ -156,6 +157,9 @@ func NewService(deps Deps) (*Service, error) {
 	if deps.Notification == nil {
 		deps.Notification = NewNoopNotificationPublisher(deps.Logger)
 	}
+	if deps.Diplomail == nil {
+		deps.Diplomail = NewNoopDiplomailPublisher(deps.Logger)
+	}
 	if deps.Policy == nil {
 		policy, err := NewPolicy()
 		if err != nil {