Fix Alembic auto-generation

You need to import `via.models` in `env.py` so that Alembic migration
auto-generation works.

via/migrations/env.py

@@ -4,6 +4,7 @@
 from alembic import context
 from sqlalchemy import engine_from_config, pool
 
+from via import models
 from via.db import Base
 
 # this is the Alembic Config object, which provides