feat: add SQLAlchemy storage adapter

[Maneesh-Relanto]

Closes the critical 'BLOCKING PRODUCTION USE' gap by providing a persistent SQL-backed storage backend.

What's new

src/rbac/storage/sqlalchemy_adapter.py (new)

• SQLAlchemyStorage class extending BaseStorage
• Full CRUD for Users, Roles, Permissions, Resources, RoleAssignments
• SQLAlchemy 2.x declarative ORM with 6 tables: rbac_users, rbac_roles, rbac_permissions, rbac_resources, rbac_role_permissions (M:M junction), rbac_role_assignments
• Proper indexes on domain, email, resource_type columns
• JSON columns for attributes/conditions/metadata (portable across DBs)
• Soft-delete for Users, Roles, Resources (status='deleted')
• Cascade deletes: deleting a user/role removes its assignments
• Circular hierarchy detection for role parent updates
• Thread-safe scoped sessions with auto commit/rollback
• initialize() / dispose() lifecycle methods
• Compatible connection kwargs for SQLite and pooled DBs
• Supports: SQLite (:memory: and file), PostgreSQL, MySQL

src/rbac/storage/init.py

• Export SQLAlchemyStorage with graceful ImportError guard (SQLAlchemy optional)

requirements.txt

• Uncommented sqlalchemy>=2.0.0 as an active dependency
• Left psycopg2-binary and pymysql commented (install per DB driver)

tests/test_sqlalchemy_storage.py (new)

• 50 tests across 7 test classes using sqlite:///:memory:
• TestLifecycle, TestUserCRUD, TestRoleCRUD, TestPermissionCRUD, TestResourceCRUD, TestRoleAssignments, TestUtility, TestPersistence
• Persistence test uses tmp_path file DB to verify write → new instance → read
• All 50 tests pass in < 1 second