fix(stats): resolve day boundaries in the reader's timezone

Day keys are written per reader but read back using UTC, the database server's CURRENT_DATE, or
the server process's local year, so streaks and day-based achievements were evaluated against the
wrong day. Route every lookup through one resolved timezone on the evaluation context.

Install tzdata so the container honours TZ for date and subprocesses. Node already resolved TZ
through ICU, so no reading data was affected by the missing package.

Closes #1084

docker-compose.yml

@@ -23,6 +23,7 @@ services:
       SETUP_BOOTSTRAP_TOKEN: ${SETUP_BOOTSTRAP_TOKEN?required}
       APP_URL: ${APP_URL?required}
       CLIENT_URL: ${CLIENT_URL:-${APP_URL?required}}
+      TZ: ${TZ:-UTC}
       PUID: ${PUID:-1000}
       PGID: ${PGID:-1000}
       NODE_MAX_OLD_SPACE_SIZE: ${NODE_MAX_OLD_SPACE_SIZE:-auto}