Configures stable constraint naming so autogeneration produces clean diffs.
Alembic uses the sync psycopg driver while the runtime app uses asyncpg.
Also fixes a .gitignore bug caught during this task: the bare 'models/'
rule for the ML weights volume was matching backend/app/models/ (Python
package). Anchored all volume rules to repo root (/images/, /import/,
/downloads/, /models/, /postgres_data/, /redis_data/).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
