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/__init__.py

@@ -2,7 +2,14 @@
 
 from app.models.tenant import Tenant
 from app.models.user import User, UserRole
-from app.models.device import Device, DeviceGroup, DeviceTag, DeviceGroupMembership, DeviceTagAssignment, DeviceStatus
+from app.models.device import (
+    Device,
+    DeviceGroup,
+    DeviceTag,
+    DeviceGroupMembership,
+    DeviceTagAssignment,
+    DeviceStatus,
+)
 from app.models.alert import AlertRule, NotificationChannel, AlertRuleChannel, AlertEvent
 from app.models.firmware import FirmwareVersion, FirmwareUpgradeJob
 from app.models.config_template import ConfigTemplate, ConfigTemplateTag, TemplatePushJob