feat: add risk-based prioritization engine

[ChaudhariPiyush15]

Description

Implements Issue #143: Risk-Based Prioritization Engine.

Backend

• Added risk_score calculation based on severity, category, ML score, and reachability
• Preserved existing ML ranking behavior
• Added risk_score persistence in the database
• Exposed risk_score through findings API responses

Frontend

• Added risk score support to TypeScript models
• Added risk score display and sorting in findings UI

Evidence Pack

• Added prioritized findings reporting

Database

• Added risk_score column
• Added migration support for existing databases

Closes #143

[github-actions]

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> **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 #

## What this PR does



## 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



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### Frontend



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### New dependencies



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### Database / schema changes



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## Testing

**How did you test this?**



**Checklist**

- [ ] Tested locally end-to-end (upload ZIP or GitHub URL → scan → findings returned correctly)
- [ ] 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



## Screenshots (if UI changed)