## Summary

Simplifies lecture-video instruction handling so `Thread.instructions`
remains the persisted assistant-instruction snapshot, while
lecture-video formatting guidance is added only when building run
instructions.

## Changes

- Store `assistant.instructions` on new lecture-video threads instead of
leaving `thread.instructions` unset.
- For existing lecture-video threads missing instructions, backfill
`thread.instructions` from the assistant snapshot on first use.
- Build lecture-video `Run.instructions` from the stored thread snapshot
plus current runtime formatting guidance.
- Stop returning `latest_run.instructions` as the thread instruction
source in thread details, so the UI sees the effective thread prompt
rather than the run-expanded prompt.
- Remove the one-off `m11_enable_latex_for_lecture_video_assistants`
command and migration helper, since Alembic now handles the remaining
`use_latex` backfill.
- Update lecture-video run-instruction tests to assert that runtime
instructions use the stored thread snapshot, not updated assistant
instructions.