[Feature] : Debate Replay & Performance Analytics

## Problem
After completing a debate, users currently have no way to review how they performed. They cannot replay the debate, analyze which arguments were strong or weak, or track their improvement over time. This makes debates feel like one-time interactions instead of a learning experience.

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## Proposed Solution
Add a **Debate Replay & Performance Analytics** feature that allows users to revisit past debates and get simple, useful feedback on their performance.

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## Core Features (MVP)

### 1. Debate Replay
Users should be able to open a past debate and:
- View the full transcript with timestamps
- See which speaker is talking (user or opponent)
- Jump to any part of the debate using timestamps

### 2. Performance Metrics
Show basic statistics for each phase (Opening, Cross-Exam, Closing):
- Word count
- Score (from existing AI judge or scoring logic)

Overall metrics:
- Total words spoken
- Best performing phase

### 3. Replay Page
A new route:
/debates/:debateId/replay
This page should show:
- Transcript on one side
- Performance metrics on the other

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## Backend Changes
- Add a new collection: `debate_replays`
- Store:
- debateId
- transcript with timestamps
- per-phase stats
- overall stats

### New API Endpoints
GET /api/debates/:debateId/replay
GET /api/users/replay-history

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## Frontend Changes
Create:
- `DebateReplay.tsx`
- `ReplayPlayer.tsx`
- `PerformanceMetrics.tsx`

Update:
- Add a **Replay** button on saved debates or profile page

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## Why This Feature Is Important
- Users can learn from their previous debates
- Encourages repeated usage and improvement
- Makes DebateAI feel more like a serious skill-building platform