fix(dev): stop BookStack inheriting Plane's DATABASE_URL

The dev stack shares one stack.env across all services via env_file, so
BookStack inherited Plane's DATABASE_URL (postgresql://...@postgres_plane).
Laravel prioritizes DATABASE_URL over the individual DB_* vars, so BookStack
connected to Plane's Postgres and 500'd with "could not find driver" (the
LSIO image ships pdo_mysql, not pdo_pgsql). Pin DATABASE_URL to BookStack's
own MariaDB DSN in its environment block.

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

stacks/dev/docker-compose.yml

@@ -57,6 +57,9 @@ services:
       - TZ
       - APP_URL=https://docs.ginnoir.com
       - APP_KEY
+      # stack.env leaks Plane's DATABASE_URL; Laravel prioritizes it over DB_* and
+      # would connect BookStack to postgres_plane. Pin it to BookStack's own MariaDB.
+      - DATABASE_URL=mysql://bookstack:bookstack_d3v_pass@mariadb_bookstack:3306/bookstack
       - DB_HOST=mariadb_bookstack
       - DB_PORT=3306
       - DB_USER=bookstack