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/maintenance_window.py

@@ -20,6 +20,7 @@ class MaintenanceWindow(Base):
     device_ids is a JSONB array of device UUID strings.
     An empty array means "all devices in tenant".
     """
+
     __tablename__ = "maintenance_windows"
 
     id: Mapped[uuid.UUID] = mapped_column(