Add Langfuse-based LLM observability (#86)

* Add Langfuse-based LLM observability

* Document Langfuse configuration

* Don't hardcode model in use

config/initializers/langfuse.rb

@@ -0,0 +1,9 @@
+require "langfuse"
+
+if ENV["LANGFUSE_PUBLIC_KEY"].present? && ENV["LANGFUSE_SECRET_KEY"].present?
+  Langfuse.configure do |config|
+    config.public_key = ENV["LANGFUSE_PUBLIC_KEY"]
+    config.secret_key = ENV["LANGFUSE_SECRET_KEY"]
+    config.host = ENV["LANGFUSE_HOST"] if ENV["LANGFUSE_HOST"].present?
+  end
+end