fix(backup): smaller forgejo dump chunks with per-chunk retries

Use 4m split chunks and up to 8 retries per chunk to survive
intermittent kubectl cp/exec websocket EOF on large dumps.

tools/cmd/forgejo-backup

@@ -135,10 +135,27 @@ else
   chunk_dir="$(mktemp -d)"
   refresh_prod_pod
   parts="$(kubectl exec --request-timeout=120 -n "${FORGEJO_NAMESPACE}" "${PROD_POD}" -c gitea -- \
-    sh -c "rm -f /tmp/forgejo-backup-part-*; split -b 8m '${DUMP_REMOTE}' /tmp/forgejo-backup-part- && ls /tmp/forgejo-backup-part-*")"
+    sh -c "rm -f /tmp/forgejo-backup-part-*; split -b 4m '${DUMP_REMOTE}' /tmp/forgejo-backup-part- && ls /tmp/forgejo-backup-part-*")"
   for part in ${parts}; do
     base="$(basename "${part}")"
-    copy_from_pod "${part}" "${chunk_dir}/${base}" || { bad "forgejo dump" "chunk copy failed: ${base}"; exit 1; }
+    local_chunk="${chunk_dir}/${base}"
+    if [[ -f "${local_chunk}" && -s "${local_chunk}" ]]; then
+      ok "forgejo dump" "reuse cached chunk ${base}"
+      continue
+    fi
+    chunk_ok=0
+    for chunk_try in 1 2 3 4 5 6 7 8; do
+      if copy_from_pod "${part}" "${local_chunk}"; then
+        chunk_ok=1
+        ok "forgejo dump" "chunk ${base} (${chunk_try})"
+        break
+      fi
+      sleep $((chunk_try * 3))
+    done
+    if [[ "${chunk_ok}" -ne 1 ]]; then
+      bad "forgejo dump" "chunk copy failed: ${base}"
+      exit 1
+    fi
   done
   cat "${chunk_dir}"/forgejo-backup-part-* > "${DUMP_PLAIN}"
   rm -rf "${chunk_dir}"