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>
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backend/app/services/mail_processor.py

@@ -12,12 +12,10 @@ from email.mime.text import MIMEText
 from email.mime.multipart import MIMEMultipart
 from email.utils import formatdate, make_msgid
 from typing import List, Dict, Any, Optional, Tuple
-from datetime import datetime
 import logging
 from aioimaplib import aioimaplib
 
 from app.models.database_models import MailAccount, MailProtocol
-from app.core.config import settings
 
 logger = logging.getLogger(__name__)