fix(deploy): support a non-standard S3 port for the pgBackRest endpoint

The S3 endpoint is a host only; some providers publish a separate, non-443
port. Add an optional PGBACKREST_S3_PORT (default 443) alongside the endpoint
host, wired through the prod overlay, write-prod-env.sh and both prod workflows,
and clarify in the docs that the endpoint variable takes the host alone.

deploy/docker-compose.prod.yml

@@ -61,6 +61,7 @@ services:
       PGBACKREST_REPO1_PATH: /pgbackrest
       PGBACKREST_REPO1_S3_URI_STYLE: path
       PGBACKREST_REPO1_S3_ENDPOINT: ${PGBACKREST_S3_ENDPOINT:-}
+      PGBACKREST_REPO1_S3_PORT: ${PGBACKREST_S3_PORT:-443}
       PGBACKREST_REPO1_S3_BUCKET: ${PGBACKREST_S3_BUCKET:-}
       PGBACKREST_REPO1_S3_REGION: ${PGBACKREST_S3_REGION:-}
       PGBACKREST_REPO1_S3_KEY: ${PGBACKREST_S3_KEY:-}