Set the OpenAI-compatible endpoint from the settings panel

Pointing OPENAI_BASE_URL at a local llama-server or Ollama sends every gpt-*
call there instead of to OpenAI, and litellm reports that as a bare
"Connection Error" — which reads like a network problem even though the host
can reach api.openai.com fine. Deploying a .env that still carries the local
endpoint reproduces it on the server, with nothing in the message naming the
culprit.

Add OPENAI_BASE_URL and OPENAI_API_BASE to the settings allowlist so the
endpoint can be corrected without editing .env and recreating the container.
Both are listed because litellm reads OPENAI_API_BASE while the OpenAI SDK
reads OPENAI_BASE_URL, so fixing one can leave the other in effect. Neither is
a secret, so the panel shows the current value.

No cache invalidation is needed alongside this: litellm keys its client cache
on the endpoint, so a changed value takes effect on the next call.

Document the trap where it will be read — a commented template in .env.example
and a note in the Lightsail deploy guide about carrying a local .env over.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/.env.example

@@ -2,6 +2,12 @@
 DEFAULT_MODEL=gpt-5.4
 # OpenAI
 OPENAI_API_KEY=sk-...
+# OpenAI互換エンドポイントに向ける場合だけ設定する（ローカルの llama-server / Ollama など）。
+# 設定したままにすると、gpt-* のモデルを選んでも接続先がそちらのままになり、
+# 本家OpenAIには繋がらない（litellm は OPENAI_API_BASE、OpenAI SDK は OPENAI_BASE_URL を見る）。
+# サーバーにデプロイする際はこの2行を消し忘れないこと。GUIの設定画面からも変更できる。
+# OPENAI_API_BASE=http://localhost:11434/v1
+# OPENAI_BASE_URL=http://localhost:11434/v1
 # Anthropic
 ANTHROPIC_API_KEY=sk-ant-...
 # Ollama (ローカル) — APIキー不要、モデル名は ollama/llama3 形式で指定