docs: move admin panel details to dedicated docs/{en,cn,ja}/admin-panel.md

The READMEs now carry a short blurb + link, matching the existing
per-topic docs (docker.md, ai-providers.md, ...). Removes the ~22-line
inline section and the duplicated data/settings.json mentions.

docs/en/admin-panel.md

@@ -0,0 +1,24 @@
+# Admin Panel
+
+Instead of hand-editing `.env`, you can manage server settings in a web admin panel.
+
+## Enabling the panel
+
+1. Set the `ADMIN_PASSWORD` environment variable (leave unset to disable the panel).
+2. Visit `/admin` and sign in.
+
+## What you can configure
+
+1. **Models** — add providers with their API keys and model lists, using the same UI as the in-app model settings. Saved models become server-side models available to all users, merged with any `AI_MODELS_CONFIG` / `ai-models.json` from your environment at request time (the panel does not modify those env files).
+2. **Other sections** — access codes, generation parameters, features, observability, and quota. Saved settings are written to `data/settings.json` and apply immediately — no restart needed (a few settings such as Langfuse and DynamoDB are marked "Restart Required").
+
+## Precedence
+
+Settings saved in the panel override environment variables, which override built-in defaults. Removing a saved value falls back to the environment variable.
+
+## Notes
+
+- Secrets are stored in plaintext in `data/settings.json` (file mode 600). Keep the file private.
+- On serverless platforms (Vercel, Cloudflare Workers) there is no persistent disk, so the panel is read-only — configure via environment variables there.
+- With Docker, the `data/` directory is persisted via the volume in `docker-compose.yml`.
+- `NEXT_PUBLIC_*` variables are baked in at build time and cannot be changed in the panel.