Subscribe to an OPML, not just import one

A feed whose body sniffs as OPML is treated as a subscription list and
re-read on every scan, as iPodderX did. Listed feeds become real config
entries grouped under it, inherit its settings, land in one nested folder,
and are scanned in the same run.

When a feed leaves the OPML: removed if nothing was downloaded, kept and
flagged otherwise, so a downloaded file is never orphaned.

folder_for sanitized the whole folder string and would have flattened the
nesting; each segment is sanitized separately now, and a traversal still
cannot escape the download directory. Db::memory() also runs migrate(),
which it did not, so a migration-only column passed tests while missing in
production.

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

src/web.rs

@@ -155,6 +155,10 @@ struct FeedRow {
     allow_explicit: bool,
     auto_download: bool,
     max_new_per_check: Option<usize>,
+    /// The OPML subscription this feed came from, if any.
+    group: Option<String>,
+    /// In a group, but the OPML no longer lists it. Kept because it has downloads.
+    orphaned: bool,
     schedule: Option<String>,
     /// The feed's own override in minutes, so the UI need not re-parse the string.
     schedule_mins: Option<u64>,
@@ -184,6 +188,8 @@ async fn feeds(State(state): State<WebState>) -> Result<Json<Vec<FeedRow>>, ApiE
             allow_explicit: feed.allow_explicit,
             auto_download: feed.auto_download,
             max_new_per_check: feed.max_new_per_check,
+            group: feed.group.clone(),
+            orphaned: s.orphaned,
             schedule: feed.schedule.clone(),
             schedule_mins: feed
                 .schedule
@@ -271,7 +277,7 @@ mod tests {
 
     fn feed(url: &str) -> crate::config::Feed {
         crate::config::Feed {
-            url: url.into(), folder: None, schedule: None, keywords: vec![], allow_explicit: false,
+            url: url.into(), folder: None, group: None, schedule: None, keywords: vec![], allow_explicit: false,
             auto_download: true, max_new_per_check: None, username: None,
             password: None, password_env: None,
         }
@@ -722,6 +728,7 @@ async fn import_opml(
             crate::config::Feed {
                 url,
                 folder: None,
+                group: None,
                 schedule: None,
                 keywords: vec![],
                 allow_explicit: false,