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/unit/test_vpn_subnet.py

@@ -1,7 +1,10 @@
 """Unit tests for VPN subnet allocation and allowed-IPs validation."""
 
 import pytest
-from app.services.vpn_service import _allocate_subnet_index_from_used, _validate_additional_allowed_ips
+from app.services.vpn_service import (
+    _allocate_subnet_index_from_used,
+    _validate_additional_allowed_ips,
+)
 
 
 class TestSubnetAllocation: