Remove legacy scripts and single-tenant support

- Deleted the migration script `migrate-to-multitenant.sh` as it is no longer needed.
- Removed `rebuild-venv.sh`, `restore.sh`, `run-backup.sh`, and other related scripts that are not compatible with the new multi-tenant architecture.
- Updated `restart.sh`, `start.sh`, `stop.sh`, `update.sh`, and other scripts to eliminate references to single-tenant deployment, ensuring they only support multi-tenant configurations.

Co-authored-by: Copilot <copilot@github.com>

install.sh

@@ -176,7 +176,6 @@ Options:
     --skip-cleanup-old            Do not run cleanup-old.sh after install
     --cleanup-old-remove-cron     Also remove matching cron entries during old-system cleanup
     --multitenant                Use docker-compose-multitenant.yml (default)
-    --singletenant               Use docker-compose.yml
   --legacy-db-name <name>       Legacy database name (default: $LEGACY_DB_NAME)
   --legacy-mongo-uri <uri>      Legacy Mongo URI (default: $LEGACY_MONGO_URI)
   --legacy-system-dir <path>    Optional old system directory to remove after migration
@@ -207,10 +206,6 @@ parse_args() {
                 COMPOSE_FILE="docker-compose-multitenant.yml"
                 shift
                 ;;
-            --singletenant)
-                COMPOSE_FILE="docker-compose.yml"
-                shift
-                ;;
             --legacy-db-name)
                 LEGACY_DB_NAME="$2"
                 shift 2