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/tests/test_backup_scheduler.py

@@ -1,7 +1,7 @@
 """Tests for dynamic backup scheduling."""
 
 import pytest
-from unittest.mock import AsyncMock, patch, MagicMock
+from unittest.mock import MagicMock
 
 from app.services.backup_scheduler import (
     build_schedule_map,