Production carries seven matchmaking_queue rows marked `matched` from May and
June, each pointing at a game that finished months ago. The previous commit
started honouring `matched` rows so the hand-off survives a re-queue — which,
without this, would have sent those players straight into a three-month-old
finished match.

liveClaimedMatch() now verifies the referenced match is real, still playable,
and actually belongs to the caller before acting on it, and deletes the row on
the spot when it is not. Self-healing, so the existing rows need no manual
cleanup. handleStatus also deletes only the row it consumed rather than every
row for that player, and returns opponent_id alongside the colour.

Covered by e2e section 17: a stale claim is refused and cleared, queueing past
one works normally, and a genuinely live claim is still delivered with the
correct opposite colour.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>