feat(build): make summer new apps admin-ready

- NewOptions.AdminSecret: summer new writes SUMMER_ADMIN__JWT__SECRET
  to .env (still 0600), generated from 32 crypto/rand bytes when empty
  and validated as at least 32 .env-safe characters
- .env.example gets an empty SUMMER_ADMIN__JWT__SECRET= placeholder
  with a comment; the secret never reaches the printed next steps
- The next steps and the generated CLAUDE.md explain the admin at
  /backend: make:admin-controller, build, migrate and
  summer admin:create --email you@example.com --superuser
- Goldens use a fixed, non-secret admin secret fixture
- Scaffolding, installation and users-and-permissions docs updated

docs/setup/installation.md

@@ -48,7 +48,7 @@ Set them on the `bash` side of the pipe, for example `curl -fsSL https://summerc
 The generated application brings its own database and configuration:
 
 - `compose.yaml` runs PostgreSQL 16, published on `127.0.0.1:54320` only, so it does not clash with a PostgreSQL already on port 5432.
-- `.env` holds the matching `SUMMER_DATABASE__DSN` and a freshly generated `SUMMER_APP__KEY`. It is excluded by `.gitignore`; `.env.example` lists the same variables.
+- `.env` holds the matching `SUMMER_DATABASE__DSN`, a freshly generated `SUMMER_APP__KEY` and a random `SUMMER_ADMIN__JWT__SECRET` for the admin. It is excluded by `.gitignore`; `.env.example` lists the same variables.
 - `config/` holds `app.yaml`, `database.yaml`, `http.yaml` and `storage.yaml`, enough for `summer serve` and `route:list`.
 
 From `sm-demo-app`, start the database, build the binary, run the migrations and serve:
@@ -68,6 +68,14 @@ curl http://127.0.0.1:8080/
 
 It returns `{"message":"Hello from SummerCMS","plugin":"golem15.demo"}`. Stop the database with `docker compose down`, or `docker compose down -v` to delete its data as well.
 
+`summer serve` prints the URL to open and the admin status. The admin lives at `http://127.0.0.1:8080/backend` once a plugin registers an admin controller; until then the status line reads `admin: off` and names the steps. When the admin is there, create the first administrator:
+
+```sh
+summer admin:create --email you@example.com --superuser
+```
+
+[Admin controllers](../backend/admin-controllers.md) shows how to add a controller, and [Users and permissions](../backend/users-and-permissions.md) how administrators sign in.
+
 ## Create an application without the script
 
 When you already have a framework checkout, or want to read the code before running anything, do the script's steps yourself. Clone the framework into `summercms`, install the tool from it and run `summer new` in the directory next to the checkout: