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/server/contract_test.go

@@ -155,6 +155,18 @@ var requestBodyStubs = map[string]map[string]any{
 		"subject": "Contract test subject",
 		"body": "Contract test body",
 	},
+	"userMailSendAdmin": {
+		"target": "user",
+		"recipient_user_id": pathParamStubs["user_id"],
+		"subject": "Contract test admin subject",
+		"body": "Contract test admin body",
+	},
+	"adminDiplomailSend": {
+		"target": "user",
+		"recipient_user_id": pathParamStubs["user_id"],
+		"subject": "Contract test admin subject",
+		"body": "Contract test admin body",
+	},
 }
 
 // TestOpenAPIContract is the top-level OpenAPI contract test. It