Dev impersonation requires pre-seeded accounts — create unknown names on demand

**Status: enhancement**

## Current behavior
`POST /api/auth/dev/impersonate` (development-only) 404s when the requested `steamName` has no existing `players` row. Local multi-client testing therefore requires a seeding step first (e.g. `seed-speedrun-players.ts`) before any account can be impersonated.

## Proposal
On a `steamName` miss, create the player row on the fly (ToS pre-accepted so throwaway accounts skip the prompt) and log in as it — making local testing zero-setup: launch two clients, impersonate any two names, play. `playerId` (uuid) misses should still 404, since creating a row for a specific uuid is meaningless. The endpoint is already gated to `NODE_ENV=development`, so production exposure is unchanged.

Busca el handler POST /api/auth/dev/impersonate y la búsqueda de jugadores que realiza, y después inspecciona seed-speedrun-players.ts para comprobar los campos del jugador y el comportamiento del inicio de sesión. La tarea estará completa cuando se cree un steamName desconocido con los ToS aceptados previamente y se inicie sesión, mientras que la ausencia de playerId siga devolviendo 404; verifica ambos recorridos mediante el endpoint de desarrollo.

Escrito por el modelo de indexación a partir del texto del issue.