Introduces a long-lived `pid` cookie (365 d) as persistent player identity
alongside the existing 24 h `sid` session cookie.

On first visit, a player row is upserted into `players`. On round end, a
transaction inserts into `games` and updates aggregates (best_attempts on wins
only, streak counters, last_played_at). DB failures are caught and logged —
the game keeps serving.

Adds a 3×2 'Your best' stats panel and a public top-10 scoreboard (fewest
best_attempts ASC, earliest last_played_at as tie-breaker; identifiers
truncated to 8 chars; current player highlighted).

Infrastructure: adds `pg` dependency, Dockerfile now installs npm deps before
copying server.mjs, docker-compose forwards all provisioned TXT_GAME_DB_*
vars and joins the postgres_postgres network for container-to-container reach.

Task: d1c49d59-57bd-4eba-9e22-f25a04157ad4