Add --model override to the CLI generate command

[alimaandev]

Problem

The CLI generate command (shortube/main.py) always uses LLM_MODEL from settings; power users must edit .env to try a different model per run.

Task

Add --model (and optionally --provider/--temperature) options to python -m shortube.main generate that override settings for that run only.

Acceptance criteria

• --model overrides the script model without touching .env
• Default behaviour (settings) is unchanged
• Help text documents the new options

Files

• shortube/main.py