Fix all 33 ruff linting errors causing CI failures

- Remove unused imports (F401) across 17 files
- Fix f-strings without placeholders (F541) in 3 files
- Add noqa: E712 to SQLAlchemy == True comparisons (valid ORM pattern)
- Preserve alembic side-effect import with noqa: F401

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/99c3a25f-3479-473d-ac95-9faaa0ddd55b

backend/app/models/database_models.py

@@ -3,7 +3,6 @@ Database models for the multi-tenant POP3 forwarder application.
 """
 
 from datetime import datetime
-from typing import Optional
 from sqlalchemy import (
     Column,
     Integer,