Atomic mission-fill for kiosk registration (fixes overfill race)

- dev/migrations/001-innodb.sql: convert pqs_queue + pqs_mission to InnoDB.
- includes/queue.php: pqs_enqueue_player() seats a player in the next open
  mission (creating one if needed), serialized with GET_LOCK and wrapped in a
  transaction; capacity judged by real COUNT(*), numplayers rewritten from it
  so the counter self-heals and can't drift.
- callsign.php: normal registration path now calls pqs_enqueue_player instead
  of the non-atomic read-check-insert.

Verified on the local stack: 140 concurrent registrations -> 24 missions,
zero overfill, zero counter drift.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dev/migrations/001-innodb.sql

@@ -0,0 +1,12 @@
+-- Migration 001: convert the two mission-fill tables to InnoDB.
+--
+-- Why: the atomic enqueue (see includes/queue.php) wraps the capacity-check +
+-- seat-take in a transaction so the two writes commit or roll back as a unit.
+-- MyISAM silently ignores transactions, so these must be InnoDB. The rest of
+-- the schema is left as-is.
+--
+-- Safe and reversible; run once against the pqs database:
+--     mysql -u pqs -ppqs pqs < dev/migrations/001-innodb.sql
+
+ALTER TABLE pqs_mission ENGINE=InnoDB;
+ALTER TABLE pqs_queue   ENGINE=InnoDB;