Add explicit model imports to Alembic env.py

## Overview

Update `alembic/env.py` to explicitly import all model modules. This is a prerequisite for the models package restructuring.

## Background

Currently, `env.py` only imports a subset of models:

```python
import ai.backend.manager.models.agent # noqa
import ai.backend.manager.models.kernel # noqa
import ai.backend.manager.models.keypair # noqa
import ai.backend.manager.models.vfolder # noqa
import ai.backend.manager.models.event_log # noqa
import ai.backend.manager.models.rbac_models # noqa
```

When `models/__init__.py` stops eagerly importing all modules, Alembic needs explicit imports to register tables with metadata.

## Tasks

- [ ] Add explicit imports for all model modules in `alembic/env.py`
- [ ] Consider using `pkgutil.iter_modules()` for automatic discovery

## Acceptance Criteria

- [ ] `alembic upgrade head` works correctly
- [ ] `alembic revision --autogenerate` detects all tables
- [ ] No regression in existing migrations

JIRA Issue: BA-3618