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.

cli/cmd/sentryctl/cmd_dashboards_test.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+func TestExtractAPIFlagDefault(t *testing.T) {
+	apiURL, rest := extractAPIFlag([]string{"abc123"}, func(string) string { return "" })
+	if apiURL != defaultAPIURL {
+		t.Fatalf("apiURL = %q, want default %q", apiURL, defaultAPIURL)
+	}
+	if !reflect.DeepEqual(rest, []string{"abc123"}) {
+		t.Fatalf("rest = %v", rest)
+	}
+}
+
+func TestExtractAPIFlagOverride(t *testing.T) {
+	apiURL, rest := extractAPIFlag([]string{"--api", "http://custom:9090", "abc123"}, func(string) string { return "" })
+	if apiURL != "http://custom:9090" {
+		t.Fatalf("apiURL = %q", apiURL)
+	}
+	if !reflect.DeepEqual(rest, []string{"abc123"}) {
+		t.Fatalf("rest = %v, want [abc123] (flag pair stripped)", rest)
+	}
+}
+
+func TestCmdDashboardsMissingSubcommand(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	code := cmdDashboards(nil, &stdout, &stderr)
+	if code != 1 {
+		t.Fatalf("code = %d, want 1", code)
+	}
+}
+
+func TestCmdDashboardsGetMissingID(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	code := cmdDashboards([]string{"get"}, &stdout, &stderr)
+	if code != 1 {
+		t.Fatalf("code = %d, want 1", code)
+	}
+}