fix: survive an unconfigured identity provider

Auth.js refuses to build a provider with no issuer, and that refusal
takes down the whole auth layer — including reading a session that
already exists. A missing or misspelled AUTH_AUTHENTIK_ISSUER would
therefore lock everyone out of an otherwise healthy application, and
explain itself only as a stack trace in the logs.

The provider is now registered only when its three settings are present.
Sessions stay readable either way, and the sign-in page says which
variables are missing instead of offering a button that fails.

Found by the first CI run, which has no .env to inherit from: every
signed-in test failed at once, looking exactly like a broken cookie.
The local suite had been passing on variables Playwright was quietly
inheriting from the development environment — so the E2E server is now
given explicit placeholders rather than whatever happens to be around.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012cSY9pVhZmJUKNN7wf1Myd

app/login/page.tsx

@@ -1,7 +1,7 @@
 import Image from 'next/image';
 import { redirect } from 'next/navigation';
 
-import { auth, signIn } from '@/lib/auth';
+import { auth, isAuthentikConfigured, signIn } from '@/lib/auth';
 
 export const metadata = { title: 'Connexion — ITA ITO' };
 
@@ -49,6 +49,16 @@ export default async function LoginPage({
           </p>
         ) : null}
 
+        {!isAuthentikConfigured ? (
+          <p
+            role="alert"
+            className="mt-8 rounded-[var(--radius-md)] border border-[var(--danger)] bg-[var(--danger-tint)] px-4 py-3 text-sm text-[var(--danger)]"
+          >
+            La connexion n’est pas configurée&nbsp;: <span className="font-mono">AUTH_AUTHENTIK_ID</span>,{' '}
+            <span className="font-mono">AUTH_AUTHENTIK_SECRET</span> et{' '}
+            <span className="font-mono">AUTH_AUTHENTIK_ISSUER</span> doivent être renseignés.
+          </p>
+        ) : (
         <form
           className="mt-8"
           action={async () => {
@@ -65,6 +75,7 @@ export default async function LoginPage({
             Se connecter avec {providerName}
           </button>
         </form>
+        )}
       </div>
     </main>
   );