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/postgres/migrations/00001_init.sql

@@ -700,7 +700,7 @@ CREATE TABLE diplomail_messages (
     ),
     CONSTRAINT diplomail_messages_kind_sender_chk CHECK (
         (kind = 'personal' AND sender_kind = 'player') OR
-        (kind = 'admin'    AND sender_kind IN ('admin', 'system'))
+        (kind = 'admin'    AND sender_kind IN ('player', 'admin', 'system'))
     ),
     CONSTRAINT diplomail_messages_broadcast_scope_chk
         CHECK (broadcast_scope IN ('single', 'game_broadcast', 'multi_game_broadcast'))