[BUG] Replace legacy query.filter_by call

## Location of the Bug
* **File/Feature:** ```admin.py```, ```api.py```, ```auth.py```, ```main.py```, ```mfa.py```, ```polls.py```, ```test_admin.py```, ```test_auth.py```, ```test_main.py```, ```test_mfa.py```
* **Version:** ```v1.9```

## Description of the Bug
The code uses legacy calls that have been replaced by newer ones. Notably, query.filter_by() is extensively used throughout the code base. Perform research, investigate, and deploy a solution. Additionally, look for any other legacy calls in the code and replace those as well.

## Intended Functionality
Keep all functionality of the application the same.

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## Acceptance Criteria & Solution Requirements
*To close this issue, the following must be met:*
- [ ] 1. Application runs as expected (bug is resolved without breaking existing features).
- [ ] 2. Code quality metrics are maintained, and linting has been considered in development.
- [ ] 3. Testing metrics are maintained, and tests have been updated/added wherever relevant during development.
- [ ] 4. Documentation has been updated to reflect the changes made during development.
- [ ] 5. Any other requirements that are specific to this issue

The issue lists ten files (admin.py, api.py, auth.py, main.py, mfa.py, polls.py, and their test files) where query.filter_by() calls need replacement. First, examine the SQLAlchemy usage in these files to understand the current patterns. Run existing tests to ensure functionality remains unchanged after replacements. Look for other legacy SQLAlchemy calls as instructed.

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