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/routers/metrics.py

@@ -65,6 +65,7 @@ async def _check_tenant_access(
     if current_user.is_super_admin:
         # Re-set tenant context to the target tenant so RLS allows the operation
         from app.database import set_tenant_context
+
         await set_tenant_context(db, str(tenant_id))
         return
     if current_user.tenant_id != tenant_id: