The unauthenticated POST /validate endpoint passed a client-controlled
filename directly into os.path.join(TMP_DIR, filename). Because
os.path.join discards the base when the second argument is absolute, and
no traversal/separator checks existed, a caller could write uploaded
content to arbitrary locations in the Lambda's writable area (/tmp),
poisoning the shared pyQuARC cache (CACHE_DIR=/tmp) or filling the disk.

Changes:
- Add safe_upload_path(): keep only a sanitized basename of the client
  filename and write into a fresh per-request /tmp/uploads/<uuid>/
  directory, with a post-resolution containment check. Neutralizes
  absolute paths, ../ traversal, nested paths, and cross-request
  collisions in warm containers. Also fixes a crash when filename is
  omitted (path.join(TMP_DIR, None) -> TypeError).
- Move wrap_inputs() inside try/finally so the per-request AUTH_TOKEN env
  var is always cleared and uploads are always removed, preventing token
  leakage into a subsequent request in a warm container.
- Stop echoing auth_key and raw file content back in the response params.
- Return only the uploaded file's basename in results, never the server
  path.
- Throttle the public API stage (rate 20/burst 40) as defense in depth.

CWE-22, CWE-73.