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/app.html

@@ -4,6 +4,7 @@
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
 			<link rel="icon" href="/favicon.png" type="image/png" />
+		<link rel="alternate" type="application/rss+xml" title="Bibdle RSS Feed" href="/feed.xml" />
 		%sveltekit.head%
 	</head>
 	<body data-sveltekit-preload-data="hover">