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 <noreply@anthropic.com>

src/routes/entries.ts

@@ -1,13 +1,6 @@
 import { Context } from "hono";
 import { Env, FeedMetadata, EmailData } from "../types";
-
-function escapeHtml(str: string): string {
-  return str
-    .replace(/&/g, "&")
-    .replace(/</g, "&lt;")
-    .replace(/>/g, "&gt;")
-    .replace(/"/g, "&quot;");
-}
+import { escapeHtml } from "../utils/html";
 
 export async function handle(c: Context): Promise<Response> {
   const env = c.env as unknown as Env;