fix(matchmaking): self-heal a missing session instead of 401

[ChronoFinale]

Summary

Fixes the "Session not found" (401) that strands an authenticated player in matchmaking (#32).

A player's session is an in-memory cache of their players row. It's wiped on a server restart and reaped by the EMQX disconnect webhook for non-lobby players — but the player's JWT is stateless and outlives it. The matchmaking routes looked the session up with a strict getSession() and threw Session not found when it was absent, with no recovery path (the client never re-auths on a still-valid token).

This routes the lookup through a shared ensureSession() that rebuilds the session from the durable player record on a miss — the same self-healing pattern /auth/refresh and the auth endpoints already use. A player who genuinely doesn't exist in the DB is still rejected (404).

Changes

• add ensure-session.ts (reuses the existing dbPlayerToSessionInit mapper — no duplicated logic)
• use ensureSession() in /queue, /matches/:id/start, /matches/:id/result
• export dbPlayerToSessionInit so the helper can share it
• regression tests: heals a known player to 200, rejects an unknown player with 404

Why this is safe / cheap

• Not a security change — the JWT already proves auth; ban and ToS checks are unaffected (bans are read fresh from the DB, not the session).
• Cheap — the warm path is still an in-memory map lookup; the DB is only touched on a cache-miss (one primary-key read), after which it's cached again.
• Consistent — 5 auth endpoints and /auth/refresh already self-heal this exact way; matchmaking was the inconsistent gate.

Scope

This is the minimal, tested fix for the reported symptom. The same hasActiveSession-or-deny pattern also exists in the EMQX auth webhook (so MQTT reconnection can't self-heal either) — that, plus client-side re-auth on a 401, is the deeper root fix. Those are noted in #32 as proposed follow-ups since they touch a broader boundary.

Refs #32

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