Add only what is a feed or links one; refuse the rest (#102)

Adding an address looked for the feed a web page links and, finding none, added the address
as it was: every check then failed, and the sidebar called it a feed that had moved. cnn.com
is one; its page links no feed. find_feed replaces feed_behind_page: the address is added if
it is a feed or an OPML list, the feed its page links if it is a web page that links one (and
that is a feed), and otherwise the add is refused with the reason, from the web page (400, the
dialog stays open) and from `ipx add`.

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

src/web.rs

@@ -1282,8 +1282,11 @@ async fn add_feed(
     Json(body): Json<NewFeed>,
 ) -> Result<Json<serde_json::Value>, ApiError> {
     let mut cfg = (*state.ctx.cfg()).clone();
-    // Before the duplicate check, so a site's page finds the feed someone already has.
-    let url = crate::feed::feed_behind_page(&state.ctx.client, &crate::feed::expand_input(&body.url)).await;
+    // Before the duplicate check, so a site's page finds the feed someone already has. What is
+    // not a feed and links none is refused here, with why, and nothing is added.
+    let url = crate::feed::find_feed(&state.ctx.client, &crate::feed::expand_input(&body.url))
+        .await
+        .map_err(|e| ApiError::bad_request(format!("{e:#}")))?;
     // Someone else may already have it. Then adding costs nothing: no second fetch, no
     // second copy on disk, just another name against the same feed.
     if let Some(existing) = crate::subscriptions(&state.ctx).await?