fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/user.py

@@ -13,6 +13,7 @@ from app.database import Base
 
 class UserRole(str, Enum):
     """User roles with increasing privilege levels."""
+
     SUPER_ADMIN = "super_admin"
     TENANT_ADMIN = "tenant_admin"
     OPERATOR = "operator"