LAN admin on 10.0.0.128 plus quick campaign mint form (#12)

Closes #11.

## Summary
- Serve Django admin on LAN IP `10.0.0.128` (not `piha.li`). Compose now publishes `0.0.0.0:8005` and passes `SHORT_ADMIN_HOSTS` into the container.
- Admin index has a campaign mint form: domain, campaign, source, metric. Save builds `https://{domain}/?utm_campaign=&utm_source=&utm_medium=` (metric) and shows a copyable short URL.
- Public `/admin/` on `piha.lc` / `piha.li` stays 404.

## Test plan
- [ ] `cd site && uv run python manage.py test`
- [ ] Recreate compose (`docker compose up --build`) so `WEB_BIND` / `SHORT_ADMIN_HOSTS` take effect
- [ ] From another LAN machine: `http://10.0.0.128:8005/admin/` (staff login) shows the mint form
- [ ] Save a link for an allowlisted domain, copy the short URL, confirm it 302s
- [ ] `https://piha.li/admin` still 404

Reviewed-on: #12

README.md

@@ -19,8 +19,9 @@ This service is standalone. Do not fold it into `monica_site`.
 named Bearer token. No token / wrong token → **401**. No tokens configured → **503**.
 `GET /<code>` never requires a token.
 
-`/admin/` is 404 on the public host (localhost only). `GET /debug/` is a mint form
-when `DEBUG=true` and never on the public short host.
+`/admin/` is 404 on the public host. Local compose serves it on `localhost` and
+`10.0.0.128` (`http://10.0.0.128:8005/admin/` from another machine on the LAN).
+`GET /debug/` is a mint form when `DEBUG=true` and never on the public short host.
 
 ## Local run
 
@@ -40,6 +41,7 @@ uv run python manage.py runserver
 # optional local admin:
 # uv run python manage.py createsuperuser
 # then http://127.0.0.1:8005/admin/  (compose) or :8000 (runserver)
+# LAN: http://10.0.0.128:8005/admin/  (compose publishes 0.0.0.0:8005)
 ```
 
 Tests (SQLite, no network):