feat(payments): trusted platform signal on the session

Record the execution platform (kind vk|telegram|direct + device subtype
ios|android|web) on each session, captured at creation and carried
gateway->backend as a trusted X-Platform header, so the upcoming
store-compliance gate has an unforgeable execution context.

- backend.sessions gains nullable platform_kind/platform_subtype columns
  (migration 00011, CHECK-constrained, jet regenerated); session.Platform
  captures them at mint, resolve returns them, middleware exposes platform(c).
  kind is derived from the establish endpoint, never a client field; the
  account-merge session mint inherits the caller's platform.
- gateway derives the platform (VK subtype from the signed vk_platform via
  vkauth, Telegram/direct best-effort from the client) and injects X-Platform
  on every authenticated backend call through the request context.
- ui submits a best-effort device subtype on the telegram/guest/email login
  requests (new FBS subtype field); VK is server-derived from the signed params.
- an unattributed session is untrusted (view-only); VK/TG self-heal on the next
  cold-start re-mint, direct/email on re-login.

Signal plumbing only, no user-visible change; X-Platform is inert until the
gate consumes it.

backend/internal/postgres/jet/backend/model/sessions.go

@@ -13,11 +13,13 @@ import (
 )
 
 type Sessions struct {
-	SessionID  uuid.UUID `sql:"primary_key"`
-	AccountID  uuid.UUID
-	TokenHash  string
-	Status     string
-	CreatedAt  time.Time
-	LastSeenAt *time.Time
-	RevokedAt  *time.Time
+	SessionID       uuid.UUID `sql:"primary_key"`
+	AccountID       uuid.UUID
+	TokenHash       string
+	Status          string
+	CreatedAt       time.Time
+	LastSeenAt      *time.Time
+	RevokedAt       *time.Time
+	PlatformKind    *string
+	PlatformSubtype *string
 }

backend/internal/postgres/jet/backend/table/sessions.go

@@ -17,13 +17,15 @@ type sessionsTable struct {
 	postgres.Table
 
 	// Columns
-	SessionID  postgres.ColumnString
-	AccountID  postgres.ColumnString
-	TokenHash  postgres.ColumnString
-	Status     postgres.ColumnString
-	CreatedAt  postgres.ColumnTimestampz
-	LastSeenAt postgres.ColumnTimestampz
-	RevokedAt  postgres.ColumnTimestampz
+	SessionID       postgres.ColumnString
+	AccountID       postgres.ColumnString
+	TokenHash       postgres.ColumnString
+	Status          postgres.ColumnString
+	CreatedAt       postgres.ColumnTimestampz
+	LastSeenAt      postgres.ColumnTimestampz
+	RevokedAt       postgres.ColumnTimestampz
+	PlatformKind    postgres.ColumnString
+	PlatformSubtype postgres.ColumnString
 
 	AllColumns     postgres.ColumnList
 	MutableColumns postgres.ColumnList
@@ -65,29 +67,33 @@ func newSessionsTable(schemaName, tableName, alias string) *SessionsTable {
 
 func newSessionsTableImpl(schemaName, tableName, alias string) sessionsTable {
 	var (
-		SessionIDColumn  = postgres.StringColumn("session_id")
-		AccountIDColumn  = postgres.StringColumn("account_id")
-		TokenHashColumn  = postgres.StringColumn("token_hash")
-		StatusColumn     = postgres.StringColumn("status")
-		CreatedAtColumn  = postgres.TimestampzColumn("created_at")
-		LastSeenAtColumn = postgres.TimestampzColumn("last_seen_at")
-		RevokedAtColumn  = postgres.TimestampzColumn("revoked_at")
-		allColumns       = postgres.ColumnList{SessionIDColumn, AccountIDColumn, TokenHashColumn, StatusColumn, CreatedAtColumn, LastSeenAtColumn, RevokedAtColumn}
-		mutableColumns   = postgres.ColumnList{AccountIDColumn, TokenHashColumn, StatusColumn, CreatedAtColumn, LastSeenAtColumn, RevokedAtColumn}
-		defaultColumns   = postgres.ColumnList{StatusColumn, CreatedAtColumn}
+		SessionIDColumn       = postgres.StringColumn("session_id")
+		AccountIDColumn       = postgres.StringColumn("account_id")
+		TokenHashColumn       = postgres.StringColumn("token_hash")
+		StatusColumn          = postgres.StringColumn("status")
+		CreatedAtColumn       = postgres.TimestampzColumn("created_at")
+		LastSeenAtColumn      = postgres.TimestampzColumn("last_seen_at")
+		RevokedAtColumn       = postgres.TimestampzColumn("revoked_at")
+		PlatformKindColumn    = postgres.StringColumn("platform_kind")
+		PlatformSubtypeColumn = postgres.StringColumn("platform_subtype")
+		allColumns            = postgres.ColumnList{SessionIDColumn, AccountIDColumn, TokenHashColumn, StatusColumn, CreatedAtColumn, LastSeenAtColumn, RevokedAtColumn, PlatformKindColumn, PlatformSubtypeColumn}
+		mutableColumns        = postgres.ColumnList{AccountIDColumn, TokenHashColumn, StatusColumn, CreatedAtColumn, LastSeenAtColumn, RevokedAtColumn, PlatformKindColumn, PlatformSubtypeColumn}
+		defaultColumns        = postgres.ColumnList{StatusColumn, CreatedAtColumn}
 	)
 
 	return sessionsTable{
 		Table: postgres.NewTable(schemaName, tableName, alias, allColumns...),
 
 		//Columns
-		SessionID:  SessionIDColumn,
-		AccountID:  AccountIDColumn,
-		TokenHash:  TokenHashColumn,
-		Status:     StatusColumn,
-		CreatedAt:  CreatedAtColumn,
-		LastSeenAt: LastSeenAtColumn,
-		RevokedAt:  RevokedAtColumn,
+		SessionID:       SessionIDColumn,
+		AccountID:       AccountIDColumn,
+		TokenHash:       TokenHashColumn,
+		Status:          StatusColumn,
+		CreatedAt:       CreatedAtColumn,
+		LastSeenAt:      LastSeenAtColumn,
+		RevokedAt:       RevokedAtColumn,
+		PlatformKind:    PlatformKindColumn,
+		PlatformSubtype: PlatformSubtypeColumn,
 
 		AllColumns:     allColumns,
 		MutableColumns: mutableColumns,