Fix: Add posthog lib to requirement for avoid opentelemetry issue

Add "posthog==5.4.0" lib to requirements to avoid chromadb issue for lib versions incompatibility with that lib (chromadb  requiere posthog>=2.4.0,<6.0.0 ) which produce error in opentelemetry.

backend/requirements.txt

@@ -51,6 +51,7 @@ langchain-community==0.3.26
 
 fake-useragent==2.1.0
 chromadb==0.6.3
+posthog==5.4.0
 pymilvus==2.5.0
 qdrant-client==1.14.3
 opensearch-py==2.8.0