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/httpclient.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"time"
+)
+
+var httpClient = &http.Client{Timeout: 30 * time.Second}
+
+// httpGetJSON GETs path and prints the pretty-printed JSON response to
+// stdout, or the error body/status to stderr. Shared by dashboards/alerts
+// list and get, which otherwise differ only in path and resource name.
+func httpGetJSON(baseURL, path string, stdout, stderr io.Writer) int {
+	resp, err := httpClient.Get(baseURL + path)
+	if err != nil {
+		fmt.Fprintf(stderr, "request failed: %v\n", err)
+		return 1
+	}
+	defer resp.Body.Close()
+	return printJSONResponse(resp, stdout, stderr)
+}
+
+// httpPostFileJSON reads file (a JSON document, e.g. an exported
+// dashboard or a rule definition) and POSTs it to path as-is -- no
+// reshaping, since the file's shape already matches what the endpoint
+// expects (the same JSON the web UI's export button and POST /rules
+// produce/accept respectively). This is what makes "apply" the seed of a
+// future Terraform provider: one JSON contract, multiple callers.
+func httpPostFileJSON(baseURL, path, file string, stdout, stderr io.Writer) int {
+	body, err := os.ReadFile(file)
+	if err != nil {
+		fmt.Fprintf(stderr, "reading %s: %v\n", file, err)
+		return 1
+	}
+	resp, err := httpClient.Post(baseURL+path, "application/json", bytes.NewReader(body))
+	if err != nil {
+		fmt.Fprintf(stderr, "request failed: %v\n", err)
+		return 1
+	}
+	defer resp.Body.Close()
+	return printJSONResponse(resp, stdout, stderr)
+}
+
+func printJSONResponse(resp *http.Response, stdout, stderr io.Writer) int {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		fmt.Fprintf(stderr, "reading response: %v\n", err)
+		return 1
+	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		var errResp errorResponseBody
+		if json.Unmarshal(body, &errResp) == nil && errResp.Error != "" {
+			fmt.Fprintf(stderr, "request failed: %s\n", errResp.Error)
+		} else {
+			fmt.Fprintf(stderr, "request failed: status %d\n", resp.StatusCode)
+		}
+		return 1
+	}
+	var pretty bytes.Buffer
+	if json.Indent(&pretty, body, "", "  ") == nil {
+		stdout.Write(pretty.Bytes())
+	} else {
+		stdout.Write(body)
+	}
+	fmt.Fprintln(stdout)
+	return 0
+}