Alembic autogenerate missing imports + duplicate GIST index for GeoAlchemy2 columns

[antosubash]

Fixed in #87 — simple_module_db.migrations.render_item now adds import fastapi_users_db_sqlalchemy.generics and import geoalchemy2 to the autogenerate imports set when those types are rendered. See framework/db/simple_module_db/migrations.py:122-125.

The remaining items (duplicate GIST index dedup, cross-module FK autogen) weren't addressed and overlap with #86; leaving those as the scope of #86.