Process Session Messages

Process all stored messages in a session that were previously stored with process_messages=false.

**Authentication Required**: Bearer token, API key, or session token

This endpoint allows you to retroactively process messages that were initially stored 
without processing. Useful for:
- Processing messages after deciding you want them as memories
- Batch processing large conversation sessions
- Re-processing sessions with updated AI models

**Processing Behavior**:
- Only processes messages with status 'stored_only' or 'pending'
- Uses the same smart batch analysis (every 15 messages)
- Respects existing memory creation pipeline