Dependency hardening: pin minimum safe dependency versions

Hi, thanks for manga-image-translator. 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):

• transformers: pin >= 4.51.0 (e.g. CVE-2025-3263)

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