feat: per-project backup paths

[Philip-Larsen-Donnelly]

Scopes backup and restore paths to a per-instance subdirectory to support multiple instances on the same host.

• Adds BACKUP_DIR ?= ./backups/$(PROJECT_NAME) to the Makefile, replacing the shared ./backups default. Can be overridden by passing a custom BACKUP_DIR on the command line (e.g. BACKUP_DIR=/mnt/nas/backups/prod PROJECT_NAME=prod make backup)
• Passes BACKUP_DIR into the docker compose environment via BACKUP_COMPOSE_CMD so the compose file can interpolate it
• Updates all four volume mounts in stacks/backup/docker-compose.yml from the hardcoded ./backups to ${BACKUP_DIR}

Backups for an instance named prod will now land in ./backups/prod/, by default, instead of ./backups/.