Matchmaking returns "Session not found" (401) for a valid JWT after a transient MQTT disconnect, with no recovery path

# Matchmaking returns "Session not found" (401) for a player with a valid JWT, with no recovery path

## Summary

A player who is correctly authenticated (valid JWT) can be permanently locked out of matchmaking with `Session not found` (401) after a single transient MQTT disconnect. Retrying does not help, because nothing recreates the in-memory session and the still-valid JWT means the client never re-authenticates. The only workaround is a full client restart.

## Root cause

The in-memory session and the stateless JWT have decoupled lifetimes, and the matchmaking route is the only flow that requires the session strictly instead of self-healing.

1. **Sessions are in-memory only.** `src/state/index.ts` — `export const sessions = new Map()`. No persistence, no rehydration on startup.

2. **JWT auth never checks for a session.** `src/middleware/authenticate.ts` only runs `verifyJwt(token)` and sets `req.player`. A valid token passes even when the session is gone.

3. **A non-lobby disconnect reaps the session immediately.** `src/features/emqx/emqx.route.ts`:

```js
async function releasePlayerLobbyOrSession(clientid) {
const session = getSession(clientid)
if (!session) return
leaveAllQueues(clientid)
if (session.lobbyCode) {
await startGracePeriod(clientid) // lobby members get a grace period
} else {
removeSession(clientid) // everyone else is removed on the spot
}
}
```

The MQTT `clientid` is the `playerId` (the client sends `player_id` as the MQTT client id), so `getSession(clientid)` / `removeSession(clientid)` resolve the real session. A player who is matchmaking — and therefore not yet in a lobby — has their session deleted the instant EMQX reports `client.disconnected`.

4. **Matchmaking requires the session strictly.** `src/features/matchmaking/matchmaking.route.ts`:

```js
const session = getSession(req.player.playerId)
if (!session) throw new AppError('Session not found', 401)
```

By contrast, ~5 other endpoints call `ensureSession()` (`src/features/auth/auth.service.ts`), which rebuilds the session from the DB when it's missing. Matchmaking is the asymmetric one that does not self-heal.

## Reproduction

1. Authenticate (Steam) — session created, MQTT connected.
2. Drop the MQTT connection briefly (ordinary network blip). The client default is `reconnect = false`, so it does not auto-reconnect.
3. EMQX fires `client.disconnected`; since the player isn't in a lobby, the server calls `removeSession` immediately.
4. Click matchmake → `POST /api/matchmaking/queue` with the still-valid JWT → `getSession` returns nothing → **`Session not found` (401)**.
5. Retrying never recovers: the JWT is still valid, so the client doesn't re-auth, and nothing else recreates the session.

## Impact

A single transient disconnect strands an authenticated player. From the user's side it looks like matchmaking is simply broken; the only fix they have is to fully quit and relaunch the game (which forces a fresh `/auth` and recreates the session).

## Suggested fixes (in order of smallest blast radius)

1. **Make matchmaking self-heal like the other routes** — use `ensureSession(req.player.playerId)` instead of strict `getSession`. A DB-backed (Steam-authed) player would transparently get their session rebuilt instead of a 401. This is the minimal, consistent fix.
2. **Don't immediately reap non-lobby sessions on disconnect** — give them the same grace period lobby members get, so a brief blip doesn't destroy the session.
3. **(Client-side, optional)** Treat a `401 Session not found` as "re-authenticate, then retry" rather than surfacing it as a terminal error.

## Notes / unverified

The mechanism above is confirmed against the code end-to-end (in-memory map, stateless JWT, immediate non-lobby reap, strict matchmaking lookup, `clientid == playerId`). What is *not* independently confirmed is that a given user report was caused by this exact path versus another session-loss trigger (e.g. a server restart/redeploy wiping the in-memory map, or a grace-period expiry). All of those funnel into the same end state — valid JWT, no session, strict 401 — so the fixes apply regardless.

Bắt đầu trong src/features/matchmaking/matchmaking.route.ts tại lần tra cứu getSession nghiêm ngặt, sau đó so sánh với ensureSession trong src/features/auth/auth.service.ts. Tái hiện request thiếu session được mô tả trong issue và xác minh rằng JWT hợp lệ có thể khôi phục session được DB hỗ trợ thay vì nhận Session not found (401).

Do mô hình lập chỉ mục viết ra từ nội dung của issue.