Refine SlideNote generation and Studio workflows

[Cat-blizzard]

Summary

• Refactor note generation, media handling, and build artifacts.
• Fix output cleanup safety, study-pack model reuse, vision endpoint routing, and the legacy notes API.
• Align Studio, setup scripts, CI, and documentation with the current CLI.

Verification

• python -m pytest -q: 337 passed.
• First-run and textbook-index smoke flows passed.
• Branch CI passed: https://github.com/Cat-blizzard/SlideNote/actions/runs/36468171016