Add a OIDC login

OIDC login is not really useful at the moment, because we cannot do anything with tokens returned by a Keycloak provider. But once ActivityPods 2.0 will be released with a full OIDC provider, the following may be useful:

Add `auth-astro` integration to astro.config.mjs like this:

```js
import { defineConfig } from 'astro/config';
import mdx from '@astrojs/mdx';
import sitemap from '@astrojs/sitemap';
import auth from "auth-astro";
import node from "@astrojs/node";

// https://astro.build/config
export default defineConfig({
site: 'https://example.com',
integrations: [mdx(), sitemap(), auth()],
output: 'server',
adapter: node({
mode: "standalone"
})
});
```

Create a auth.config.js file with this content:

```js
import Keycloak from '@auth/core/providers/keycloak';
import { loadEnv } from 'vite';

const env = loadEnv('production', process.cwd(), '');

export default {
providers: [
Keycloak({
clientId: env.OIDC_CLIENT_ID,
clientSecret: env.OIDC_CLIENT_SECRET,
issuer: env.OIDC_ISSUER,
// https://github.com/nextauthjs/next-auth/blob/main/packages/core/src/providers/oauth.ts#L145-L152
// profile(profile) {
// console.log('profile', profile)
// return {}
// },
})
],
};
```

The Header component can look like this:

```astro
---
import HeaderLink from './HeaderLink.astro';
import { SITE_TITLE } from '../consts';
import { SignIn, SignOut } from 'auth-astro/components'
import { getSession } from 'auth-astro/server';
const session = await getSession(Astro.request);
---

{SITE_TITLE}

```

Start by reviewing astro.config.mjs and the proposed auth.config.js, then inspect the Header component and the existing Astro server setup. Configure the Keycloak provider with the OIDC environment variables, expose sign-in/sign-out and session display in the header, and verify that the website can authenticate and use the returned session.

Written by the indexing model from the issue text.