Remove denormalized short_name from Exercise model

[nbars]

Summary

Exercise.short_name is currently denormalized — it exists on both Exercise and ExerciseConfig. This was done to avoid breaking SQLAlchemy queries, Docker resource naming, and SSH routing during the ExerciseConfig migration.

Goal

Remove short_name from the Exercise model and always resolve it through the ExerciseConfig relationship (exercise.config.short_name). This eliminates the need to keep both in sync on rename.

What needs to change

• Remove Exercise.short_name column and add a proxy property
• Update all SQLAlchemy queries that filter on Exercise.short_name to join through ExerciseConfig
• Update Docker resource naming (core/instance.py, core/image.py) to use exercise.config.short_name
• Update SSH routing (view/api.py) to query via ExerciseConfig
• DB migration to drop the column
• Update the rename logic in exercise_edit_config (no longer needs to update Exercise rows)

Context

Introduced in commit ef478f8 (ExerciseConfig migration). See comment on Exercise.short_name in webapp/ref/model/exercise.py.