chore(litellm): retire Langfuse — drop success/failure callbacks (redundant + crash-prone)

Langfuse's ClickHouse member spewed ~94 GB of unrotated logs and filled ana-docker's
root disk (took the fleet host to 100%, 28/48 containers unhealthy). Its trace UI was
redundant with LiteLLM's native logging — store_prompts_in_spend_logs:true already
captures full prompts/responses/tokens/cost/latency at :4000/ui — and nothing used its
unique trace-grouping/eval features (it only received flat gateway success_callbacks).
Removed the callbacks (gateway observability stays fully native) and tore down the
6-container langfuse stack + volumes on ana-docker. Re-add the callbacks if it returns.

stacks/litellm/conf/config.yaml

@@ -334,9 +334,8 @@ litellm_settings:
   # File mounted at /app/strip_empty_tools.py; reference is module.instance,
   # resolved relative to this config's directory.
   callbacks: ["strip_empty_tools.strip_empty_tools_instance"]
-  # --- Langfuse trace export (live 2026-06-05). Full prompt/completion +
-  #     reasoning + tok-derivable latency traces ship to the Langfuse stack on
-  #     ana-docker (project "gateway"). Keys + host in .env. The gateway and
-  #     every consumer stay pointed here — this callback is the whole upgrade. ---
-  success_callback: ["langfuse"]
-  failure_callback: ["langfuse"]
+  # Langfuse trace export RETIRED 2026-06-20 (operator). Its ClickHouse member spewed
+  # ~94 GB of unrotated logs and filled ana-docker's disk; the trace UI was redundant
+  # with LiteLLM's native spend_logs (store_prompts_in_spend_logs: true → full
+  # prompts/responses/tokens/cost at :4000/ui). Gateway observability stays fully
+  # native. Re-add success_callback/failure_callback here if Langfuse ever returns.