Permanently eliminate the "$X variable not set" warning, and fix Caddy auto-HTTPS on private IPs

Two separate leaks were causing the same cosmetic-but-annoying warning to
survive every previous fix attempt:

1. Compose's own `secrets:` block reads the referenced file's *content*
   as part of its config model, and applies the same interpolation
   warning to it — even with DB_PASSWORD fully removed from every ${VAR}
   and --env-file path. Switched from Compose's native `secrets:` to plain
   bind mounts at the same /run/secrets/* paths: a bind mount only ever
   touches the file's path, never its content, so it's immune. (Verified
   this precisely with an isolated repro before rolling it out — the two
   mechanisms behave differently even though they look equivalent.)

2. caddy's `env_file: .env.staging` (a leftover from the since-removed
   basic-auth setup) loaded the *raw*, unfiltered env file directly,
   bypassing deploy.sh/backup.sh/restore.sh's filtered-copy mechanism
   entirely. Caddy only ever needed SITE_DOMAIN; switched to passing that
   one value directly instead of the whole file.

Also fixed Caddyfile.staging: the site address had no explicit scheme, so
Caddy's automatic-HTTPS logic still applied to a private IP (registering
its own internal CA and redirecting HTTP->HTTPS) — not the "plain HTTP
only" behavior I'd assumed and told the user earlier. Prefixed with
`http://` to genuinely disable automatic HTTPS for this LAN-only box.

Verified end-to-end: fresh deploy with dollar-sign DB passwords produces
zero warnings, serves HTTP 200 on plain http:// with no redirect, and the
DB connection genuinely authenticates.

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

deploy/restore.sh

@@ -12,7 +12,14 @@ cd "$REPO_ROOT"
 ENV_FILE=".env.staging"
 export ENVIRONMENT=staging
 
-COMPOSE="docker compose -p bookstore-staging -f docker-compose.yml -f docker-compose.staging.yml --env-file ${ENV_FILE}"
+# See deploy.sh for why: Compose warns on any $-shaped value in --env-file
+# even when unused, so DB_PASSWORD/DB_ROOT_PASSWORD are filtered out of the
+# copy Compose actually sees.
+COMPOSE_ENV_FILE="$(mktemp)"
+trap 'rm -f "$COMPOSE_ENV_FILE"' EXIT
+grep -Ev '^(DB_PASSWORD|DB_ROOT_PASSWORD)=' "$ENV_FILE" > "$COMPOSE_ENV_FILE"
+
+COMPOSE="docker compose -p bookstore-staging -f docker-compose.yml -f docker-compose.staging.yml --env-file ${COMPOSE_ENV_FILE}"
 
 echo "!! this OVERWRITES the staging database and uploads !!"
 read -r -p "type 'restore' to continue: " confirm