Add Alerts webapp UI (ticket 020)

New Alerts panel: lists rules with condition summaries and an enabled
toggle, and a condition-type-specific editor (Immediate/Threshold/NewValue/
Ratio) with client-side validation before save, reusing FilterBar's query
input styling for query-based conditions. Adds a test-notification endpoint
(POST /api/settings/alerts/test) so an ntfy topic/server can be verified
without saving a rule first, plus a docs section and help icons matching
ticket 021's pattern.

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

src/dto.rs

@@ -259,6 +259,16 @@ pub struct UpdateAlertRuleRequest {
     pub sink: AlertSink,
 }
 
+/// Request body for `POST /api/settings/alerts/test` (ticket 020): delivers
+/// a test notification to an ntfy topic/server without requiring a saved
+/// rule. `server` defaults to `https://ntfy.sh` when absent, same as
+/// `AlertSink::Ntfy`.
+#[derive(Debug, Deserialize)]
+pub struct TestAlertNotificationRequest {
+    pub topic: String,
+    pub server: Option<String>,
+}
+
 /// Summary of one profile for `GET /api/settings/profiles` (ticket 011):
 /// just the name and declaration/dashboard counts the Presets panel needs to
 /// render a list, rather than the full param/metric declaration bodies.