Add audit logging to model switcher

[Trishanthsai]

Before opening: make sure there is an issue tracking this work, and link it below. PRs without a linked issue may be closed without review.

Linked issue

Closes #817

What this PR does

Adds audit logging when the default AI model is changed through the settings API. Model switch events are now logged with the previous and new model values, improving traceability while preserving existing functionality.

Type of change

• Bug fix
• New feature
• ML model / training pipeline
• Refactor (no behaviour change)
• Documentation
• Tests only

ML tier (if applicable)

• Tier 1 — Triage
• Tier 2 — Predictive
• Tier 3 — Autonomous
• Not ML-related

Stack affected

• Backend
• Frontend
• Both

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Changes

Backend

• Added audit logging for model switch events in the settings routes.
• Logged previous and updated model values when default_model changes.
• Preserved existing settings update behaviour.

Frontend

• No frontend changes.

New dependencies

• None.

Database / schema changes

• None.

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Testing

How did you test this?

• Updated the existing settings API test.
• Verified that changing the default model still succeeds.
• Verified that an audit log entry is emitted when the model changes.

Checklist

• Tested locally
• New ML model falls back gracefully when model file is absent
• No new console.error or unhandled Python exceptions introduced
• Added or updated tests where applicable
• requirements.txt / package.json updated if new dependencies added
• New model files (.pkl, .pt, etc.) are gitignored, not committed

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Anything reviewers should focus on

Please review the audit logging implementation for model switch events and confirm that logging is emitted only when the configured default model changes.

Screenshots (if UI changed)

N/A (Backend-only changes)

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[Trishanthsai]

Hi @imDarshanGK i have raised a pr review it when u get chance thanks!!

[imDarshanGK]

@Trishanthsai Contains unnecessary changes and leftover code. Keep the PR focused on the audit logging implementation, remove the extra changes, and share a screenshot or short video demonstrating the audit logging.