fix(rss): halt future-verse leaks, correct timezone dates, fix discovery

- Cap /feed.xml at the current America/New_York calendar day so
  pre-scheduled future verses (and stray 2124 rows) can no longer
  surface at the top of the feed
- Emit DST-aware RFC 822 pubDates (correct -04:00/-05:00 offset) instead
  of a hardcoded EST offset, and parse calendar days as UTC so titles
  no longer shift by one day on UTC hosts
- Add content-addressed ETag + Last-Modified with If-None-Match /
  If-Modified-Since conditional handling
- Add atom:link rel=self, content:encoded, channel pubDate/generator/image
- Move RSS auto-discovery <link> into app.html: the root route sets
  ssr=false, so the layout <svelte:head> link never reached the raw HTML
  and feed readers could not discover /feed.xml
- Extract feed helpers into $lib/server/rss and add unit tests

src/routes/+layout.svelte

@@ -31,12 +31,6 @@
 
 <svelte:head>
 	<link rel="icon" href={favicon} />
-	<link
-		rel="alternate"
-		type="application/rss+xml"
-		title="Bibdle RSS Feed"
-		href="/feed.xml"
-	/>
 	<meta name="description" content="A Wordle-inspired daily Bible game" />
 </svelte:head>