[FEATURE] Role-Based Debate Mode (Historian / Scientist / Lawyer)

## 🚀 Feature Summary

Introduce a **Role-Based Debate Mode** where participants (human or AI) debate while assuming predefined intellectual roles such as **Historian**, **Scientist**, or **Lawyer**. Each role enforces a distinct argumentation style, improving educational value and debate structure.

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## 🎯 Motivation & Problem Statement

Currently, debates in DebateAI follow a generic argumentation style regardless of context. This limits:
- Educational depth
- Argument diversity
- Structured reasoning based on discipline-specific thinking

Role-based debating encourages:
- Evidence-driven arguments (Scientist)
- Historical context and precedents (Historian)
- Persuasive and logical framing (Lawyer)

This aligns strongly with DebateAI’s mission of improving critical thinking and structured discourse.

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## 🧠 Proposed Roles (Initial Scope)

| Role | Debate Characteristics |
|-----|------------------------|
| **Historian** | Uses historical examples, timelines, precedents |
| **Scientist** | Focuses on data, experiments, empirical evidence |
| **Lawyer** | Emphasizes persuasion, logic, rebuttals, rhetoric |

> Future roles (Philosopher, Economist, Policy Analyst) can be added later.

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## ⚙️ Proposed Implementation Plan

### Backend (Go)
- Add a `role` field to debate participants:
```go
type DebateRole string
const (
Historian DebateRole = "historian"
Scientist DebateRole = "scientist"
Lawyer DebateRole = "lawyer"
)

I am willing to submit PR.