Bug: Streaming endpoint decorator on wrong function causing 422 errors

Body:
## Description
The `/sessions/{id}/messages/stream` endpoint returns 422
validation errors because the `@router.post()` decorator is
incorrectly placed.

## Location
`backend/api/messages.py` lines 64-85

## Problem
The decorator is on `safe_json_dumps()` (line 65) instead of
`send_message_stream()` (line 85).

## Expected behavior
```python
@router.post("/sessions/{session_id}/messages/stream")
async def send_message_stream(session_id: str, request:
SendMessageRequest):

Actual behavior

@router.post("/sessions/{session_id}/messages/stream")
def safe_json_dumps(obj): # ← Wrong function!

Impact

- Streaming messages fail with 422 error
- Error: Field required: obj

Fix

Move decorator from line 64 to line 84 (before
send_message_stream)

Steps to reproduce

1. Create session
2. Send message via streaming endpoint
3. Get 422 validation error