Drop entity_fields table and remove FieldType enum, FieldRef, EntityFieldRow

The entity_fields table and related types (FieldType, FieldRef, EntityFieldRow) are no longer actively used after BA-4457 removed RBACFieldCreator/RBACFieldPurger infrastructure (BEP-1048).

Scope:

- Alembic migration to drop entity_fields table (drop indexes + drop table)

- Delete EntityFieldRow model (src/ai/backend/manager/models/rbac_models/entity_field.py)

- Remove EntityFieldRow from rbac_models/__init__.py

- Delete FieldType enum from src/ai/backend/common/data/permission/types.py

- Delete FieldRef class from src/ai/backend/manager/data/permission/id.py

- Remove FieldType re-export from src/ai/backend/manager/data/permission/types.py

- Remove EntityFieldRow from test fixtures (with_tables) in vfolder, scheduler, deployment tests

JIRA Issue: BA-4750