Look up derived feeds everywhere, not just in config

Moving OPML feeds into the database left several call sites still
searching config.toml only, so anything inside a subscription looked
unsubscribed: Download failed outright, status and the startup line
counted 3 feeds instead of 85, add could duplicate or collide with a
derived feed, and rm could not remove one.

The first grep for this missed the failing call because the method chain
spans lines; searching with newlines collapsed found all of them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AdXho5tTkjFLeUXKbEjKBh

src/web.rs

@@ -398,8 +398,12 @@ async fn add_feed(
     Json(body): Json<NewFeed>,
 ) -> Result<Json<serde_json::Value>, ApiError> {
     let mut cfg = (*state.ctx.cfg()).clone();
-    if let Some((id, _)) = cfg.feeds.iter().find(|(_, f)| f.url == body.url) {
-        return Ok(Json(serde_json::json!({ "id": id, "existing": true })));
+    // Derived feeds count as subscribed: adding one an OPML already lists would duplicate it.
+    if let Some(existing) = crate::subscriptions(&state.ctx)?
+        .into_iter()
+        .find(|s| s.cfg.url == body.url)
+    {
+        return Ok(Json(serde_json::json!({ "id": existing.id, "existing": true })));
     }
     let id = crate::add_one(&state.ctx, &mut cfg, &body.url, body.folder, body.keywords).await?;
     cfg.save(&state.config_path)?;