Add alert archiving

Alerts can be manually archived (POST /api/alerts/{id}/archive) or
unarchived (DELETE /api/alerts/{id}/archive). A background goroutine
auto-archives resolved alerts older than TERDUT_ARCHIVE_AFTER (default 7d).
GET /api/alerts hides archived alerts by default; ?archived=true shows them.

internal/api/api_test.go

@@ -357,6 +357,66 @@ func TestStats_ByHourReturnsTwentyFourSlots(t *testing.T) {
 	}
 }
 
+// ---------------------------------------------------------------------------
+// Archive
+// ---------------------------------------------------------------------------
+
+func TestArchive_RoundTrip(t *testing.T) {
+	s := newTS(t)
+
+	postWebhook(t, s, []map[string]any{{
+		"status": "resolved", "fingerprint": "arch1",
+		"labels": map[string]string{"alertname": "Archivable"},
+		"annotations": map[string]string{},
+		"startsAt": "2026-05-20T10:00:00Z", "endsAt": "2026-05-20T11:00:00Z",
+		"generatorURL": "",
+	}})
+
+	// 1. Alert appears in default list (not archived).
+	var alerts []map[string]any
+	decode(t, s.req(t, http.MethodGet, "/api/alerts", nil), &alerts)
+	if len(alerts) != 1 {
+		t.Fatalf("expected 1 alert in default list, got %d", len(alerts))
+	}
+	id := int(alerts[0]["id"].(float64))
+
+	// 2. Archive it.
+	resp := s.req(t, http.MethodPost, fmt.Sprintf("/api/alerts/%d/archive", id), nil)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("archive: expected 200, got %d", resp.StatusCode)
+	}
+	var archived map[string]any
+	decode(t, resp, &archived)
+	if archived["archived_at"] == nil {
+		t.Error("expected archived_at to be set in response")
+	}
+
+	// 3. Default list excludes it.
+	decode(t, s.req(t, http.MethodGet, "/api/alerts", nil), &alerts)
+	if len(alerts) != 0 {
+		t.Errorf("expected archived alert to be hidden, got %d results", len(alerts))
+	}
+
+	// 4. archived=true shows it.
+	decode(t, s.req(t, http.MethodGet, "/api/alerts?archived=true", nil), &alerts)
+	if len(alerts) != 1 {
+		t.Fatalf("expected 1 archived alert, got %d", len(alerts))
+	}
+
+	// 5. Un-archive.
+	resp = s.req(t, http.MethodDelete, fmt.Sprintf("/api/alerts/%d/archive", id), nil)
+	if resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("unarchive: expected 204, got %d", resp.StatusCode)
+	}
+	resp.Body.Close()
+
+	// 6. Back in default list.
+	decode(t, s.req(t, http.MethodGet, "/api/alerts", nil), &alerts)
+	if len(alerts) != 1 {
+		t.Errorf("expected unarchived alert to reappear, got %d results", len(alerts))
+	}
+}
+
 func TestStats_ByDayReturnsSevenSlots(t *testing.T) {
 	s := newTS(t)
 	resp := s.req(t, http.MethodGet, "/api/stats/alerts/by-day", nil)