Dependency hardening: pin minimum safe dependency versions

Hi, thanks for pdfGPT. Low-priority dependency-hardening note, not an active vulnerability report - a default install resolves to safe versions today.

These declarations also permit older versions with known CVEs; pinning a minimum prevents an accidental downgrade (please test compatibility):

• litellm: pin >= 1.44.8 (e.g. CVE-2024-6587)

(Low-priority dependency-hygiene note - feel free to close if not useful, happy to send a PR.)