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.

alerting/internal/queryclient/client.go

@@ -0,0 +1,75 @@
+// Package queryclient is a thin HTTP client to /api's POST /query --
+// alerting never imports querylang or talks to ClickHouse/Tantivy
+// directly, same precedent sentryctl query and the web UI's dashboard
+// panels already set: one query-execution path, reused everywhere.
+package queryclient
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+type Result struct {
+	Columns []string `json:"columns"`
+	Rows    [][]any  `json:"rows"`
+}
+
+type errorResponse struct {
+	Error string `json:"error"`
+}
+
+type Client struct {
+	baseURL string
+	http    *http.Client
+}
+
+func New(baseURL string) *Client {
+	return &Client{baseURL: baseURL, http: &http.Client{}}
+}
+
+// Query runs query (already time-range-injected by the caller, if
+// applicable) against /api's POST /query and returns the result. A
+// non-2xx response or a request/decode failure is returned as an error
+// -- callers (the evaluator) treat any error here as an evaluation
+// error, never as "condition false" (see
+// /docs/phase-3-alerting-design.md's fix 3).
+func (c *Client) Query(ctx context.Context, query, language string, timeout time.Duration) (*Result, error) {
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	body, err := json.Marshal(map[string]string{"query": query, "language": language})
+	if err != nil {
+		return nil, fmt.Errorf("encoding query request: %w", err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.baseURL+"/query", bytes.NewReader(body))
+	if err != nil {
+		return nil, fmt.Errorf("building query request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("calling api /query: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		var errBody errorResponse
+		_ = json.NewDecoder(resp.Body).Decode(&errBody)
+		if errBody.Error != "" {
+			return nil, fmt.Errorf("api /query failed (%d): %s", resp.StatusCode, errBody.Error)
+		}
+		return nil, fmt.Errorf("api /query failed with status %d", resp.StatusCode)
+	}
+
+	var result Result
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return nil, fmt.Errorf("decoding query response: %w", err)
+	}
+	return &result, nil
+}