Fix DB password corruption via Docker secrets file mechanism

The reported bug (a generated password containing "$BRjTx3tlmSpz" got
silently blanked, breaking the DB connection) is the same Compose
interpolation issue as the earlier bcrypt hash, but this time in fields
that are genuinely user-chosen and can't just be avoided by convention.

Switched DB_PASSWORD/DB_ROOT_PASSWORD to Docker's official `_FILE`
secrets convention (MARIADB_PASSWORD_FILE / WORDPRESS_DB_PASSWORD_FILE),
backed by Compose's native `secrets:` mechanism: deploy.sh writes the raw
value to secrets/<env>/db_password, and the container reads that file
directly — the value never passes through Compose's ${VAR} interpolation
at all. Verified end-to-end with an actual `$`-containing password,
including a full deploy → backup → restore → still-serving round trip.

Also fixed along the way (found while actually testing, not assumed):
- Makefile never exported ENVIRONMENT, so `make up` alone (bypassing
  deploy.sh) would have left the new secrets path unresolved.
- deploy.sh chmod'd the secret files 600, unreadable by the container's
  own UID (www-data) — fixed to 644, relying on the containing directory
  (700) to keep other host users out instead.
- backup.sh/restore.sh still called `mysqldump`/`mysql`, which don't
  exist in the mariadb:11 image under those names — renamed to
  mariadb-dump/mariadb. (This means neither script had actually
  succeeded before now; both are verified working end-to-end here.)

The supplier/payment API keys remain passed the old way — nothing reads
them yet (bookstore-core is still a stub), so there's no live bug to fix
there; noted in .env.example that the same _FILE pattern should be used
once that code exists.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

deploy/backup.sh

@@ -4,6 +4,7 @@
 set -euo pipefail
 
 ENVIRONMENT="${1:?Usage: backup.sh <staging|production>}"
+export ENVIRONMENT
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 cd "$REPO_ROOT"
 
@@ -21,7 +22,7 @@ mkdir -p "$BACKUP_DIR"
 COMPOSE="docker compose -p bookstore-${ENVIRONMENT} -f docker-compose.yml -f docker-compose.${ENVIRONMENT}.yml --env-file ${ENV_FILE}"
 
 echo "==> dumping database"
-$COMPOSE exec -T db sh -c "exec mysqldump -u\"\$MARIADB_USER\" -p\"\$MARIADB_PASSWORD\" \"\$MARIADB_DATABASE\"" \
+$COMPOSE exec -T db sh -c "exec mariadb-dump -u\"\$MARIADB_USER\" -p\"\$(cat /run/secrets/db_password)\" \"\$MARIADB_DATABASE\"" \
   | gzip > "$BACKUP_DIR/db-${TIMESTAMP}.sql.gz"
 
 echo "==> archiving uploads"