fix(bookstack): set APP_KEY so init no longer halts (docs.ginnoir.com 502)

linuxserver/bookstack refuses to start without APP_KEY:
  'The application key is missing, halting init!'
nginx never bound :80, so caddy -> bookstack:80 -> connection refused -> 502.

Key generated via 'docker run --rm --entrypoint /bin/bash
lscr.io/linuxserver/bookstack:latest appkey' on valhalla. Stored in
stacks/dev/.env per per-stack secrets policy; do NOT rotate without
invalidating existing sessions.

stacks/dev/docker-compose.yml

@@ -118,6 +118,7 @@ services:
       - PGID=1000
       - TZ=${TZ}
       - APP_URL=https://docs.ginnoir.com
+      - APP_KEY=${BOOKSTACK_APP_KEY}
       - DB_HOST=mariadb_bookstack
       - DB_PORT=3306
       - DB_USER=bookstack