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/postgres/jet/backend/table/table_use_schema.go

@@ -16,6 +16,9 @@ func UseSchema(schema string) {
 	AuthChallenges = AuthChallenges.FromSchema(schema)
 	BlockedEmails = BlockedEmails.FromSchema(schema)
 	DeviceSessions = DeviceSessions.FromSchema(schema)
+	DiplomailMessages = DiplomailMessages.FromSchema(schema)
+	DiplomailRecipients = DiplomailRecipients.FromSchema(schema)
+	DiplomailTranslations = DiplomailTranslations.FromSchema(schema)
 	EngineVersions = EngineVersions.FromSchema(schema)
 	EntitlementRecords = EntitlementRecords.FromSchema(schema)
 	EntitlementSnapshots = EntitlementSnapshots.FromSchema(schema)