Every game starts a series; a game created in a series gets its seat to
move first. Clients get the series and the first seat with the game, the
end message the series with the ended game; snapshots keep the series.
The Quarkus project moves to apps/backend and the Express sources go.
The image is built from the repo root as before, with Temurin 25, and
runs as a non-root user on port 6000. Compose passes the database as a
JDBC URL with the same MANCALA_DB_PASSWORD.
