Directory: let an admin give a blog its category

Almost no blog names a category the Directory can use, so a feed can
carry one of its own in config.toml, set by an admin in the feed's
settings and used when the feed names none. The feed's own iTunes
category still wins. The field offers the categories the Directory
already shows, so a blog about games joins Games rather than starting a
second chip. Setting it on a feed from an OPML promotes it to config, as
any other shared setting does.

Closes #10.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/config.rs

@@ -115,6 +115,10 @@ pub struct Feed {
     /// Download folder name; defaults to the sanitized feed title.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub folder: Option<String>,
+    /// The Directory's category for a feed that names none of its own, as most blogs do not.
+    /// The feed's own iTunes category wins where there is one.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub category: Option<String>,
     /// Every whitespace-separated word of a keyword must appear in the
     /// url/title/description/categories for an enclosure to be taken.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
@@ -469,7 +473,7 @@ mod tests {
         taken.insert("the-daily".to_string(), Feed {
             url: "u".into(), folder: None, group: None, media_types: None, schedule: None, keywords: vec![], allow_explicit: false,
             auto_download: true, max_new_per_check: None, username: None,
-            password: None, password_env: None,
+            password: None, password_env: None, category: None,
         });
         assert_eq!(unique_slug("The Daily", &taken), "the-daily-2");
     }
@@ -489,6 +493,7 @@ mod tests {
             username: Some("ray".into()),
             password: Some("literal".into()),
             password_env: None,
+            category: None,
         };
         assert_eq!(f.password().as_deref(), Some("literal"));