refactor: improve CORS and streaming response headers

- Add configurable CORS origins
- Remove duplicate CORS headers
- Add X-Accel-Buffering header
- Update env example file
- Clean up header configurations

.env.example

@@ -1,12 +1,13 @@
 ### Server Configuration
-#HOST=0.0.0.0
-#PORT=9621
-#NAMESPACE_PREFIX=lightrag  # separating data from difference Lightrag instances
+# HOST=0.0.0.0
+# PORT=9621
+# NAMESPACE_PREFIX=lightrag  # separating data from difference Lightrag instances
+# CORS_ORIGINS=http://localhost:3000,http://localhost:8080
 
 ### Optional SSL Configuration
-#SSL=true
-#SSL_CERTFILE=/path/to/cert.pem
-#SSL_KEYFILE=/path/to/key.pem
+# SSL=true
+# SSL_CERTFILE=/path/to/cert.pem
+# SSL_KEYFILE=/path/to/key.pem
 
 ### Security (empty for no api-key is needed)
 # LIGHTRAG_API_KEY=your-secure-api-key-here