feat(tracing): Integrate Langfuse for LLM call tracing and add documentation

docs/installation/TRACING.md

@@ -0,0 +1,42 @@
+
+# Tracing LLM Calls in Perplexica
+
+Perplexica supports tracing all LangChain and LangGraph LLM calls for debugging, analytics, and prompt transparency. You can use either Langfuse (self-hosted, private, or cloud) or LangSmith (cloud, by LangChain) for tracing.
+
+## Langfuse Tracing (Recommended for Private/Self-Hosted)
+
+Langfuse is an open-source, self-hostable observability platform for LLM applications. It allows you to trace prompts, completions, and tool calls **privately**—no data leaves your infrastructure if you self-host.
+
+### Setup
+
+1. **Deploy Langfuse**
+	- See: [Langfuse Self-Hosting Guide](https://langfuse.com/docs/self-hosting)
+	- You can also use the Langfuse Cloud if you prefer.
+
+2. **Configure Environment Variables**
+	- Add the following to your environment variables in docker-compose or your deployment environment:
+
+	  ```env
+	  LANGFUSE_PUBLIC_KEY=your-public-key
+	  LANGFUSE_SECRET_KEY=your-secret-key
+	  LANGFUSE_BASE_URL=https://your-langfuse-instance.com
+	  ```
+	- These are required for the tracing integration to work. If not set, tracing is disabled gracefully.
+
+3. **Run Perplexica**
+	- All LLM and agent calls will be traced automatically. You can view traces in your Langfuse dashboard.
+
+## LangSmith Tracing (Cloud by LangChain)
+
+Perplexica also supports tracing via [LangSmith](https://smith.langchain.com/), the official observability platform by LangChain.
+
+- To enable LangSmith, follow the official guide: [LangSmith Observability Docs](https://docs.smith.langchain.com/observability)
+- Set the required environment variables as described in their documentation.
+
+**LangSmith is a managed cloud service.**
+
+---
+
+For more details on tracing, see the respective documentation:
+- [Langfuse Documentation](https://langfuse.com/docs)
+- [LangSmith Observability](https://docs.smith.langchain.com/observability)