feat(account): deletion orchestration + step-up + gateway edge

Step-up: email accounts confirm with a mailed code (purpose=delete, no deeplink —
ConfirmByToken refuses a delete token so a stray click can't delete); platform-only
accounts type a fixed phrase (anti-impulse). Endpoints /user/delete/{request,confirm};
the confirm orchestration resigns active games, drops all-robot games, tombstones +
anonymizes the account (freeing its creds), and revokes its sessions — the tombstone is
the point of no return, the rest best-effort. Gateway account.delete.{request,confirm}
ops + fbs AccountDeleteConfirm/AccountDeleteRequestResult + branded ru/en delete email.
Integration tests cover the step-up (code + no-email) and the orchestration pieces.

backend/internal/server/handlers.go

@@ -80,6 +80,10 @@ func (s *Server) registerRoutes() {
 		// refused without disclosure, never merged).
 		u.POST("/link/email/change/request", s.handleChangeEmailRequest)
 		u.POST("/link/email/change/confirm", s.handleChangeEmailConfirm)
+		// Account deletion (legal retention, not erasure): step-up via a mailed code
+		// (email accounts) or a typed phrase (platform-only), then tombstone + free creds.
+		u.POST("/delete/request", s.handleRequestDelete)
+		u.POST("/delete/confirm", s.handleConfirmDelete)
 	}
 	if s.games != nil {
 		u.GET("/games", s.handleListGames)