Adding a site's address subscribes to the feed it links (#71, #72)

Both add paths, the CLI's and the web's, look behind the URL first: a web
page that names its feed with <link rel="alternate"> is swapped for that
feed, before the duplicate check so it finds a feed someone already has.
Before, the page itself was added and every scan failed on it.

alternate_feed_link found tags in a to_lowercase() copy and sliced the
original at those offsets; Unicode lowercasing changes some characters'
length, so a page with one before its <link> tags lost the href or
panicked off a char boundary. ASCII lowercasing keeps offsets aligned.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main.rs

@@ -606,7 +606,7 @@ async fn add(
     keywords: Vec<String>,
 ) -> Result<()> {
     let mut cfg = (*ctx.cfg()).clone();
-    let url = &feed::expand_input(url);
+    let url = &feed::feed_behind_page(&ctx.client, &feed::expand_input(url)).await;
     // Includes feeds derived from an OPML, or the same show could be added twice.
     if let Some(existing) = subscriptions(ctx).await?.iter().find(|s| feed::same_feed(&s.cfg.url, url)) {
         anyhow::bail!("already subscribed as {:?}", existing.id);