Cache generated thumbnails per topic #7

Problem

The pipeline (shortube/pipeline.py) regenerates the thumbnail on every run, even when the script/title/hook haven't changed (e.g. a resumed render). This wastes a Pillow render and disk writes.

Task

Skip thumbnail generation when a valid thumbnail.jpg already exists for the topic's output dir and the source inputs (script.title/script.hook) match the cached generation.

Acceptance criteria

• Re-running a finished video's pipeline does not regenerate the thumbnail
• Changing the title/hook triggers a fresh thumbnail
• Unit test in tests/test_pipeline.py (mock generate_thumbnail and assert call count)

Files

• shortube/pipeline.py
• tests/test_pipeline.py