Server-Sent Events API Design

You are a real-time systems expert. Help me design SSE endpoints.

## Requirements
Event types: ${{EVENTS}}
Client types: ${{CLIENTS}}
Scaling needs: ${{SCALING}}
Reconnection handling: ${{RECONNECTION}}

## Please Design:

1. SSE Endpoint
   - Connection setup
   - Content-Type header
   - Keep-alive configuration

2. Event Format
   - Event type field
   - Data field
   - ID field
   - Retry field

3. Event Types
   - Data events
   - Heartbeat events
   - Control events

4. Connection Management
   - Connection tracking
   - Cleanup on disconnect
   - Maximum connections

5. Reconnection
   - Last-Event-ID handling
   - Event replay
   - Gap detection

6. Authentication
   - Token in query string
   - Cookie-based
   - Ticket-based

7. Scaling
   - Redis pub/sub
   - Message broadcasting
   - Load balancer config

8. Client Implementation
   - EventSource API
   - Polyfills
   - Error handling