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_auth.py

@@ -11,7 +11,6 @@ These are pure function tests -- no database or async required.
 
 import uuid
 from datetime import UTC, datetime, timedelta
-from unittest.mock import patch
 
 import pytest
 from fastapi import HTTPException
@@ -83,9 +82,7 @@ class TestAccessToken:
         assert payload["role"] == "super_admin"
 
     def test_contains_expiry(self):
-        token = create_access_token(
-            user_id=uuid.uuid4(), tenant_id=uuid.uuid4(), role="viewer"
-        )
+        token = create_access_token(user_id=uuid.uuid4(), tenant_id=uuid.uuid4(), role="viewer")
         payload = verify_token(token, expected_type="access")
         assert "exp" in payload
         assert "iat" in payload