nook: add an appcast upload helper

The release pipeline had no in-repo path to put a corrected appcast
back on S3 (the backfill of 0.6.0/0.6.2 notes needed one). Reads the
same bucket env the appcast route does; refuses to upload unless every
expected release item is present.

Tests: used for the production backfill; endpoint re-verified after.

scripts/upload-appcast.ts

@@ -0,0 +1,38 @@
+// One-off: upload the backfilled appcast to the download bucket.
+// Usage: cd ~/Code/freno-dev && bun --env-file=.env /tmp/upload-appcast.ts <file>
+import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";
+import { readFileSync } from "node:fs";
+
+const [, , file] = process.argv;
+if (!file) {
+  console.error("usage: bun --env-file=.env upload-appcast.ts <appcast.xml>");
+  process.exit(1);
+}
+
+const body = readFileSync(file);
+const xml = body.toString("utf8");
+// Guard: refuse to upload without all three release descriptions present.
+for (const v of ["0.6.0", "0.6.2", "0.6.3"]) {
+  if (!xml.includes(`sparkle:shortVersionString>${v}<`)) {
+    console.error(`refusing: item for ${v} missing`);
+    process.exit(1);
+  }
+}
+
+const client = new S3Client({
+  region: process.env.AWS_REGION,
+  credentials: {
+    accessKeyId: process.env.MY_AWS_ACCESS_KEY!,
+    secretAccessKey: process.env.MY_AWS_SECRET_KEY!
+  }
+});
+
+await client.send(
+  new PutObjectCommand({
+    Bucket: process.env.VITE_DOWNLOAD_BUCKET_STRING,
+    Key: "api/TheNook/appcast.xml",
+    Body: body,
+    ContentType: "application/xml; charset=utf-8"
+  })
+);
+console.log("uploaded", file, "->", "api/TheNook/appcast.xml");