fix(slink): generate admin credentials and JWT keys on install

- Set ADMIN_EMAIL/ADMIN_PASSWORD so admin user is created on first boot
- Generate JWT keypair for authentication to work
- Run doctrine:migrations:migrate for DB schema
- Save credentials to ~/slink.creds

json/slink.json

@@ -28,14 +28,18 @@
         }
     ],
     "default_credentials": {
-        "username": null,
-        "password": null
+        "username": "admin@localhost",
+        "password": "auto-generated"
     },
     "notes": [
         {
             "text": "API runs on Caddy+PHP-FPM (port 8080), client on Node.js (port 3000).",
             "type": "info"
         },
+        {
+            "text": "Credentials are saved to `~/slink.creds`.",
+            "type": "info"
+        },
         {
             "text": "Mercure real-time notifications are not available in this bare-metal setup.",
             "type": "warning"