[grade=B urn:ump:x37xj4seku3isvffxkwmahymvjijrwinndnfpubv664spur7kogq] fix(installer): remove BACKUP_DIR readonly collision that aborted every install

readonly BACKUP_DIR (constants) collided with the BACKUP_DIR= tunable, so every
curl|bash run died with 'BACKUP_DIR: readonly variable'. Removed the readonly
constant and resolve the default (/etc/dashcaddy/backups) at end of parse_args;
--backup-dir flag still overrides. Live file + /opt copy patched identically.

dashcaddy-installer/install.sh

@@ -27,7 +27,6 @@ readonly API_DIR="${SITES_DIR}/dashcaddy-api"
 readonly DASHBOARD_DIR="${SITES_DIR}/status"
 readonly CONTAINER_NAME="dashcaddy-api"
 readonly CADDY_ADMIN_PORT=2019
-readonly BACKUP_DIR="${BACKUP_DIR:-${INSTALL_DIR}/backups}"
 readonly DEFAULT_MAX_STORAGE_BYTES=""
 
 # ---- Tunables (overridable via flags) --------------------------------------
@@ -1045,6 +1044,9 @@ parse_args() {
 
   # Normalize TLD
   if [[ -n "$TLD" && "$TLD" != .* ]]; then TLD=".$TLD"; fi
+
+  # Resolve BACKUP_DIR default when not set by the --backup-dir flag.
+  BACKUP_DIR="${BACKUP_DIR:-${INSTALL_DIR}/backups}"
 }
 
 print_help() {