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/config.rs

@@ -97,6 +97,10 @@ pub struct Feed {
     pub allow_explicit: bool,
     #[serde(default = "yes")]
     pub auto_download: bool,
+    /// Set on feeds that came from a subscribed OPML: the id of the OPML feed they
+    /// belong to. The OPML is re-read on every scan and this list kept in step.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub group: Option<String>,
     /// Overrides the global schedule for this feed. Same forms: "every 6h", "2d".
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub schedule: Option<String>,
@@ -393,7 +397,7 @@ mod tests {
 
         let mut taken = BTreeMap::new();
         taken.insert("the-daily".to_string(), Feed {
-            url: "u".into(), folder: None, schedule: None, keywords: vec![], allow_explicit: false,
+            url: "u".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,
         });
@@ -405,6 +409,7 @@ mod tests {
         let mut f = Feed {
             url: "https://x/y".into(),
             folder: None,
+            group: None,
             schedule: None,
             keywords: vec![],
             allow_explicit: false,