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.

POST /api/auth/dev/impersonate handler とその player lookup を探し、次に seed-speedrun-players.ts を調べて player fields と login behavior を確認します。未知の steamName が ToS を事前に受け入れた状態で作成され、ログインされる一方で、playerId がない場合は引き続き 404 が返されることが完了条件です。development endpoint を通じて両方のパスを検証してください。

索引モデルが issue の本文から書いたものです。