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/auth_service.py

@@ -2,8 +2,7 @@
 OAuth2 authentication service for Google and other providers.
 """
 
-from typing import Dict, Any, Optional
-from datetime import datetime
+from typing import Dict, Any
 import httpx
 from authlib.integrations.starlette_client import OAuth
 from fastapi import HTTPException, status