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/services/openbao_service.py

@@ -7,6 +7,7 @@ material never leaves OpenBao -- the application only sees ciphertext.
 
 Ciphertext format: "vault:v1:base64..." (compatible with Vault Transit format)
 """
+
 import base64
 import logging
 from typing import Optional