refactor: simplify quick-win code after review

- Make feedId required in generateRssFeed (removes dead /emails/ fallback)
- Hoist loop-invariant conditional and remove intermediate variable
- Extract normalizeAllowedSenders() so JSON and form paths share same logic
- Move escapeHtml to src/utils/html.ts for reuse by admin.ts
- Parallelize the two independent KV puts in feed creation

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/utils/html.ts

@@ -0,0 +1,7 @@
+export function escapeHtml(str: string): string {
+  return str
+    .replace(/&/g, "&")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;")
+    .replace(/"/g, "&quot;");
+}