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/contract_test.go

@@ -46,6 +46,7 @@ var pathParamStubs = map[string]string{
 	"user_id": "00000000-0000-0000-0000-000000000007",
 	"device_session_id": "00000000-0000-0000-0000-000000000008",
 	"battle_id": "00000000-0000-0000-0000-000000000009",
+	"message_id": "00000000-0000-0000-0000-00000000000a",
 	"id": "1.2.3",
 	"username": "alice",
 	"turn": "42",
@@ -149,6 +150,11 @@ var requestBodyStubs = map[string]map[string]any{
 		"user_id": pathParamStubs["user_id"],
 		"reason": "ToS violation",
 	},
+	"userMailSendPersonal": {
+		"recipient_user_id": pathParamStubs["user_id"],
+		"subject": "Contract test subject",
+		"body": "Contract test body",
+	},
 }
 
 // TestOpenAPIContract is the top-level OpenAPI contract test. It