feat(backend): add SSE streaming endpoint for real-time chat responses

[SandeepChauhan00]

🔗 Related Issue

Fixes #50

📋 Summary

Adds Server-Sent Events (SSE) streaming endpoint for real-time chat responses, providing a ChatGPT-like experience where users see responses appear word-by-word.

🐛 Problem

Currently, users wait 5-10 seconds staring at a loading spinner before seeing any response. This feels slow and unresponsive.

✅ Solution

New /api/chat/stream endpoint that:

• Streams tokens in real-time using SSE
• Shows responses word-by-word (3 words per chunk)
• Includes start, token, done, and error message types
• Maintains backward compatibility with existing /api/chat endpoint

🔄 Before & After

Before	After
Wait 5-10 seconds	Response appears in ~1 second
Full text at once	Words appear as generated
Feels slow	Feels instant (like ChatGPT)

🧪 Testing Performed

Tested with: curl "http://localhost:8000/api/chat/stream?query=EEG%20datasets"

Output shows streaming tokens appearing in real-time with start, token, and done messages.

✅ Checklist

• Backend endpoint works
• Streaming tokens appear in real-time
• Error handling included
• Backward compatible with existing endpoint
• Tested locally