fix: reduce memory usage for monthly ZIP archive flow

Replace generated archive download response with FileResponse streaming so ZIP files are not fully loaded into process memory.

Move monthly archive task output to a temporary file and upload that file to storage, and write invoice attachments into ZIP archives in chunks.

Add Celery memory guard settings, a memory-aware base task hook, and periodic /tmp cleanup for prefixed archive files older than one day.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/scripts/cleanup_tmp_files.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+set -eu
+
+TMP_DIR="${APP_TMP_CLEANUP_DIR:-/tmp}"
+MAX_AGE_DAYS="${APP_TMP_CLEANUP_MAX_AGE_DAYS:-1}"
+PREFIXES="${APP_TMP_CLEANUP_PREFIXES:-erp-fleet-archive-}"
+
+if [ ! -d "$TMP_DIR" ]; then
+  echo "TMP dir ne postoji: $TMP_DIR"
+  exit 0
+fi
+
+OLD_IFS="$IFS"
+IFS=','
+for prefix in $PREFIXES; do
+  prefix_trimmed="$(echo "$prefix" | xargs)"
+  if [ -z "$prefix_trimmed" ]; then
+    continue
+  fi
+  find "$TMP_DIR" -maxdepth 1 -mindepth 1 -name "${prefix_trimmed}*" -mtime "+${MAX_AGE_DAYS}" -print -delete
+done
+IFS="$OLD_IFS"
+
+echo "TMP cleanup dovršen za $TMP_DIR (older than ${MAX_AGE_DAYS} day(s))."