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/api/v1/endpoints/mail_accounts.py

@@ -7,7 +7,7 @@ from sqlalchemy import select, desc
 
 from app.core.database import get_db
 from app.core.deps import get_current_active_user
-from app.core.security import encrypt_credential, decrypt_credential
+from app.core.security import encrypt_credential
 from app.models.database_models import User, MailAccount
 from app.models.schemas import (
     MailAccountCreate,
@@ -52,7 +52,7 @@ async def create_mail_account(
     if len(existing_accounts) >= max_accounts:
         raise HTTPException(
             status_code=status.HTTP_402_PAYMENT_REQUIRED,
-            detail=f"Account limit reached. Upgrade your subscription to add more accounts.",
+            detail="Account limit reached. Upgrade your subscription to add more accounts.",
         )
 
     # Encrypt password