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_loghandlers.go

@@ -28,6 +28,8 @@ func (pm *ProxyManager) streamLogsHandler(c *gin.Context) {
 	c.Header("Content-Type", "text/plain")
 	c.Header("Transfer-Encoding", "chunked")
 	c.Header("X-Content-Type-Options", "nosniff")
+	// prevent nginx from buffering streamed logs
+	c.Header("X-Accel-Buffering", "no")
 
 	logMonitorId := c.Param("logMonitorID")
 	logger, err := pm.getLogger(logMonitorId)