feat: Archived alerts tab with archive/unarchive actions

Add a fourth tab (Alerts | Archived | Schedule | Users).
Archived alerts are fetched lazily on first visit using the
archived=true query param on GET /api/alerts.

Press x from the Alerts list or detail to archive an alert;
the non-archived list refreshes immediately. Press x from the
Archived list or detail to unarchive; the archived list
refreshes. Ack/unack are disabled in the Archived detail view.

New API methods: ArchiveAlert (POST), UnarchiveAlert (DELETE).
ArchivedAt field added to the Alert type.

internal/api/client.go

@@ -61,12 +61,16 @@ func (c *Client) do(req *http.Request, out any) error {
 	return nil
 }
 
-// ListAlerts fetches alerts from the server. status may be "firing", "resolved", or "" for all.
-func (c *Client) ListAlerts(status string, limit int) ([]Alert, error) {
+// ListAlerts fetches alerts. status may be "firing", "resolved", or "" for all.
+// Set archived=true to fetch only archived alerts; false returns only non-archived.
+func (c *Client) ListAlerts(status string, archived bool, limit int) ([]Alert, error) {
 	q := url.Values{}
 	if status != "" {
 		q.Set("status", status)
 	}
+	if archived {
+		q.Set("archived", "true")
+	}
 	if limit > 0 {
 		q.Set("limit", strconv.Itoa(limit))
 	}
@@ -83,6 +87,23 @@ func (c *Client) ListAlerts(status string, limit int) ([]Alert, error) {
 	return alerts, c.do(req, &alerts)
 }
 
+func (c *Client) ArchiveAlert(id int64) (*Alert, error) {
+	req, err := c.newRequest(http.MethodPost, fmt.Sprintf("/api/alerts/%d/archive", id))
+	if err != nil {
+		return nil, err
+	}
+	var alert Alert
+	return &alert, c.do(req, &alert)
+}
+
+func (c *Client) UnarchiveAlert(id int64) error {
+	req, err := c.newRequest(http.MethodDelete, fmt.Sprintf("/api/alerts/%d/archive", id))
+	if err != nil {
+		return err
+	}
+	return c.do(req, nil)
+}
+
 // GetAlertStats fetches aggregate alert counts.
 func (c *Client) GetAlertStats() (*AlertStats, error) {
 	req, err := c.newRequest(http.MethodGet, "/api/stats/alerts")