[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The code uses legacy calls that have been replaced by newer ones.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