feat: Add admin credentials config (default: admin / ScratchingPost2026!)

appsettings.json

@@ -6,5 +6,9 @@
     }
   },
   "AllowedHosts": "*",
-  "MaxUploadSizeMB": 20
+  "MaxUploadSizeMB": 20,
+  "Admin": {
+    "Username": "admin",
+    "Password": "ScratchingPost2026!"
+  }
 }