Fix nginx proxy buffering for streaming endpoints (#295)

* Fix nginx proxy buffering for streaming endpoints

- Add X-Accel-Buffering: no header to SSE endpoints (/api/events, /logs/stream)
- Add X-Accel-Buffering: no header to proxied text/event-stream responses
- Add nginx reverse proxy configuration section to README
- Add tests for X-Accel-Buffering header on streaming endpoints

Fixes #236

* Fix goroutine cleanup in streaming endpoints test

Add context cancellation to TestProxyManager_StreamingEndpointsReturnNoBufferingHeader
to ensure the goroutine is properly cleaned up when the test completes.

proxy/proxymanager_api.go

@@ -100,6 +100,8 @@ func (pm *ProxyManager) apiSendEvents(c *gin.Context) {
 	c.Header("Cache-Control", "no-cache")
 	c.Header("Connection", "keep-alive")
 	c.Header("X-Content-Type-Options", "nosniff")
+	// prevent nginx from buffering SSE
+	c.Header("X-Accel-Buffering", "no")
 
 	sendBuffer := make(chan messageEnvelope, 25)
 	ctx, cancel := context.WithCancel(c.Request.Context())