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/lib/server/rss.ts

@@ -0,0 +1,165 @@
+import type { DailyVerse } from '$lib/server/db/schema';
+
+/**
+ * The feed presents calendar days, so every timestamp is anchored to midnight
+ * in this reference timezone. Using a fixed zone (rather than the server's
+ * local timezone) keeps output stable across UTC dev machines and NY hosts.
+ */
+export const FEED_TIMEZONE = 'America/New_York';
+
+/** Number of most-recent verses exposed in the feed. */
+export const FEED_ITEM_LIMIT = 30;
+
+/** Escape the five XML predefined entities. */
+export function escapeXml(text: string): string {
+	return text
+		.replace(/&/g, '&')
+		.replace(/</g, '&lt;')
+		.replace(/>/g, '&gt;')
+		.replace(/"/g, '&quot;')
+		.replace(/'/g, '&apos;');
+}
+
+/**
+ * The current calendar date (`YYYY-MM-DD`) in the feed's reference timezone.
+ * Used to cap the feed so future/scheduled verses are never published early.
+ */
+export function todayInTimeZone(timeZone: string = FEED_TIMEZONE, now: Date = new Date()): string {
+	const parts = new Intl.DateTimeFormat('en-US', {
+		timeZone,
+		year: 'numeric',
+		month: '2-digit',
+		day: '2-digit'
+	}).formatToParts(now);
+	const get = (type: string) => parts.find((part) => part.type === type)?.value ?? '';
+	return `${get('year')}-${get('month')}-${get('day')}`;
+}
+
+/**
+ * The UTC offset in effect on a given `YYYY-MM-DD` for a timezone, formatted
+ * as `±HH:MM` (e.g. `-04:00` during EDT, `-05:00` during EST).
+ */
+export function timeZoneOffset(dateStr: string, timeZone: string = FEED_TIMEZONE): string {
+	const [year, month, day] = dateStr.split('-').map(Number);
+	// Noon UTC is safely inside the calendar day in any US offset, so DST is
+	// resolved correctly regardless of the time of day.
+	const reference = new Date(Date.UTC(year, month - 1, day, 12));
+	const name = new Intl.DateTimeFormat('en-US', {
+		timeZone,
+		timeZoneName: 'longOffset'
+	})
+		.formatToParts(reference)
+		.find((part) => part.type === 'timeZoneName')?.value;
+
+	const offset = (name ?? 'GMT+00:00').replace('GMT', '');
+	return offset === '' ? '+00:00' : offset;
+}
+
+/**
+ * Format a `YYYY-MM-DD` day as an RFC 822 / RFC 2822 timestamp at midnight in
+ * the feed's reference timezone. Always emits a valid numeric/GMT timestamp
+ * (never a bare, ambiguous abbreviation like `EST`).
+ */
+export function formatRFC822(dateStr: string, timeZone: string = FEED_TIMEZONE): string {
+	const offset = timeZoneOffset(dateStr, timeZone);
+	return new Date(`${dateStr}T00:00:00${offset}`).toUTCString();
+}
+
+/** Human-readable date that does not shift with the server's local timezone. */
+export function formatReadableDate(dateStr: string): string {
+	const [year, month, day] = dateStr.split('-').map(Number);
+	return new Date(Date.UTC(year, month - 1, day)).toLocaleDateString('en-US', {
+		month: 'long',
+		day: 'numeric',
+		year: 'numeric',
+		timeZone: 'UTC'
+	});
+}
+
+/** Wrap text in a CDATA section, splitting any embedded `]]>` safely. */
+export function cdata(text: string): string {
+	return `<![CDATA[${text.replace(/]]>/g, ']]]]><![CDATA[>')}]]>`;
+}
+
+// Helper: Format verse text (VerseDisplay + Imposter unbalanced punctuation handling)
+export function formatVerseText(text: string): string {
+	let formatted = text;
+
+	// Handle unbalanced opening/closing punctuation (from Imposter.svelte)
+	const pairs: [string, string][] = [
+		['(', ')'],
+		['[', ']'],
+		['{', '}'],
+		['"', '"'],
+		["'", "'"],
+		['\u201C', '\u201D'], // " "
+		['\u2018', '\u2019'] // ' '
+	];
+
+	// Check if text starts with opening punctuation without closing
+	for (const [open, close] of pairs) {
+		if (formatted.startsWith(open) && !formatted.includes(close)) {
+			formatted += '...' + close;
+			break;
+		}
+	}
+
+	// Check if text ends with closing punctuation without opening
+	for (const [open, close] of pairs) {
+		if (formatted.endsWith(close) && !formatted.includes(open)) {
+			formatted = open + '...' + formatted;
+			break;
+		}
+	}
+
+	// Check if text contains unbalanced opening quotes (not at start) without closing
+	for (const [open, close] of pairs) {
+		const openCount = (formatted.match(new RegExp(`\\${open}`, 'g')) || []).length;
+		const closeCount = (formatted.match(new RegExp(`\\${close}`, 'g')) || []).length;
+		if (openCount > closeCount) {
+			formatted += close;
+			break;
+		}
+	}
+
+	// Capitalize first letter if lowercase (from VerseDisplay.svelte)
+	formatted = formatted.replace(/^([a-z])/, (c) => c.toUpperCase());
+
+	// Replace trailing punctuation with ellipsis
+	// Preserve closing quotes/brackets that may have been added
+	formatted = formatted.replace(/[,:;-—]([)\]}\"\'\u201D\u2019]*)$/, '...$1');
+
+	return formatted;
+}
+
+/**
+ * Content fingerprint used as a strong ETag. Any change to the included rows
+ * (new day, edited verse text, changed reference) produces a new hash, so
+ * cached clients revalidate correctly.
+ */
+export function feedFingerprint(
+	verses: Pick<DailyVerse, 'date' | 'reference' | 'verseText'>[]
+): string {
+	let hash = 5381;
+	for (const verse of verses) {
+		const row = `${verse.date}\u0000${verse.reference}\u0000${verse.verseText}`;
+		for (let i = 0; i < row.length; i++) {
+			hash = ((hash << 5) + hash + row.charCodeAt(i)) | 0;
+		}
+	}
+	return (hash >>> 0).toString(16);
+}
+
+/**
+ * The most recent `createdAt` across the given rows, used for `Last-Modified`.
+ * Falls back to `fallback` when no row carries a timestamp.
+ */
+export function latestModification(
+	verses: Pick<DailyVerse, 'createdAt'>[],
+	fallback: Date
+): Date {
+	return verses.reduce<Date | null>((latest, verse) => {
+		if (!verse.createdAt) return latest;
+		return !latest || verse.createdAt > latest ? verse.createdAt : latest;
+	}, null) ?? fallback;
+}