Add descriptions and examples to Pydantic model fields

[tranguyeenn]

Summary

Improve Pydantic model documentation using Field() descriptions and examples.

Context

Pydantic metadata improves readability and produces clearer OpenAPI documentation.

Desired Outcome

Add meaningful descriptions and example values to model fields.

Scope

Update Pydantic models with:

• description=
• examples=
• model-level docstrings where useful

Files Likely Affected

• backend model definitions
• schema files

Acceptance Criteria

• Model fields include descriptions where appropriate
• Example values are added
• OpenAPI documentation improves

Notes

This task should not change validation behavior.