Move the ntfy alert sink to Settings, shared by all alert rules

Alert rules no longer carry their own ntfy topic/server; Settings now
holds one process-wide ntfy sink (with its own "send test notification"),
and the Alerts panel is left to just define rule conditions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/settings.rs

@@ -78,6 +78,12 @@ pub struct Settings {
     /// Alert rule definitions.
     #[serde(default)]
     pub alert_rules: Vec<AlertRule>,
+    /// Ntfy topic every alert rule delivers to; `None` until configured in Settings.
+    #[serde(default)]
+    pub ntfy_topic: Option<String>,
+    /// Ntfy server override; `None` means `https://ntfy.sh`.
+    #[serde(default)]
+    pub ntfy_server: Option<String>,
 }
 
 impl Default for Settings {
@@ -93,6 +99,8 @@ impl Default for Settings {
             },
             max_logs: None,
             alert_rules: Vec::new(),
+            ntfy_topic: None,
+            ntfy_server: None,
         }
     }
 }
@@ -333,11 +341,8 @@ mod tests {
             condition: crate::alerts::AlertCondition::Immediate {
                 query: r#"regex("error")"#.to_string(),
             },
-            sink: crate::alerts::AlertSink::Ntfy {
-                topic: "alerts".to_string(),
-                server: None,
-            },
         });
+        settings.ntfy_topic = Some("alerts".to_string());
 
         settings.save(&path).unwrap();