Phase 3: dashboards and alerting

Saved, shareable multi-panel dashboards (table/line/bar/single-stat
panels via gridstack + uPlot, global + per-panel time range, JSON
export/import) and threshold/absence alert rules with an
ok/pending/firing evaluator and webhook/Slack/PagerDuty delivery.

- New /metadata component: Postgres control-plane store for dashboards,
  panels, notification targets, alert rules/state, and delivery log --
  see docs/phase-3-dashboard-design.md for why ClickHouse's MergeTree
  family isn't a fit for this access pattern (needs real row-level
  locking and read-your-writes consistency).
- api/internal/dashboards: dashboard/panel CRUD, pure -- panel query
  execution stays client-side, reusing the existing /query endpoint.
- New /alerting service: rule/target CRUD, a ticker-driven evaluator
  (claim-then-evaluate concurrency control, transactional-outbox
  delivery, query errors and threshold zero-rows never coerced into a
  false transition) and webhook/Slack/PagerDuty delivery with
  retry/backoff. See docs/phase-3-alerting-design.md for the full
  state-machine design and the four correctness properties it
  implements.
- web: /dashboards and /alerts UIs; cli: sentryctl dashboards/alerts
  list/get/apply, seeding a future Terraform provider's JSON contract.
- hack/alert-load-test: 500 rules against real ClickHouse data, real
  measured results in docs/phase-3-runbook.md.

Five real bugs found by actually running this against a live stack
(documented in the runbook, not just fixed silently): a latent Phase 2
bug where ClickHouse rejected the timestamp format used for
earliest=/latest= queries; a "now" literal token injected into query
text; a GridStack/uPlot layout-timing race; JS's Date.parse being too
lenient to use as a timestamp-detection heuristic; a rule's "enabled"
field silently defaulting to false when omitted; and the evaluator's
claim-batch-size and worker-pool-concurrency defaulting to the same
value, causing 500 concurrently-due rules to take 125s to cycle through
instead of the configured 60s.

api/internal/queryapi/handler_test.go

@@ -50,14 +50,20 @@ func newTestHandler(sqlRunner *fakeSQLRunner, search *fakeSearchClient) *Handler
 	if search == nil {
 		search = &fakeSearchClient{}
 	}
-	return NewHandler(slog.New(slog.NewTextHandler(io.Discard, nil)), sqlRunner, search, time.Second, "*")
+	return NewHandler(slog.New(slog.NewTextHandler(io.Discard, nil)), sqlRunner, search, time.Second)
+}
+
+func newTestMux(h *Handler) *http.ServeMux {
+	mux := http.NewServeMux()
+	h.RegisterRoutes(mux)
+	return mux
 }
 
 func postQuery(t *testing.T, h *Handler, body string) *httptest.ResponseRecorder {
 	t.Helper()
 	req := httptest.NewRequest(http.MethodPost, "/query", strings.NewReader(body))
 	rec := httptest.NewRecorder()
-	h.Routes().ServeHTTP(rec, req)
+	newTestMux(h).ServeHTTP(rec, req)
 	return rec
 }
 
@@ -195,24 +201,9 @@ func TestHandleHealthz(t *testing.T) {
 	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
 	rec := httptest.NewRecorder()
 
-	h.Routes().ServeHTTP(rec, req)
+	newTestMux(h).ServeHTTP(rec, req)
 
 	if rec.Code != http.StatusOK {
 		t.Fatalf("status = %d, want 200", rec.Code)
 	}
 }
-
-func TestCORSPreflight(t *testing.T) {
-	h := newTestHandler(&fakeSQLRunner{}, nil)
-	req := httptest.NewRequest(http.MethodOptions, "/query", nil)
-	rec := httptest.NewRecorder()
-
-	h.Routes().ServeHTTP(rec, req)
-
-	if rec.Code != http.StatusNoContent {
-		t.Fatalf("status = %d, want 204", rec.Code)
-	}
-	if got := rec.Header().Get("Access-Control-Allow-Origin"); got != "*" {
-		t.Fatalf("Access-Control-Allow-Origin = %q, want *", got)
-	}
-}