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/notifystore/store.go

@@ -0,0 +1,114 @@
+// Package notifystore is pgx-backed CRUD for notification_targets.
+package notifystore
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+var ErrNotFound = errors.New("not found")
+
+type Kind string
+
+const (
+	KindWebhook   Kind = "webhook"
+	KindSlack     Kind = "slack"
+	KindPagerDuty Kind = "pagerduty"
+)
+
+func ValidKind(k Kind) bool {
+	switch k {
+	case KindWebhook, KindSlack, KindPagerDuty:
+		return true
+	default:
+		return false
+	}
+}
+
+type Target struct {
+	ID              string          `json:"id"`
+	TenantID        string          `json:"tenant_id"`
+	Name            string          `json:"name"`
+	Kind            Kind            `json:"kind"`
+	WebhookURL      string          `json:"webhook_url"`
+	PayloadTemplate *string         `json:"payload_template,omitempty"`
+	Headers         json.RawMessage `json:"headers,omitempty"`
+	Secret          *string         `json:"secret,omitempty"`
+	CreatedBy       string          `json:"created_by"`
+}
+
+type Store struct {
+	pool *pgxpool.Pool
+}
+
+func NewStore(pool *pgxpool.Pool) *Store {
+	return &Store{pool: pool}
+}
+
+func (s *Store) Create(ctx context.Context, t *Target) error {
+	t.ID = uuid.NewString()
+	if t.TenantID == "" {
+		t.TenantID = "default"
+	}
+	if t.CreatedBy == "" {
+		t.CreatedBy = "anonymous"
+	}
+	if len(t.Headers) == 0 {
+		t.Headers = json.RawMessage(`{}`)
+	}
+	_, err := s.pool.Exec(ctx, `
+		INSERT INTO notification_targets (id, tenant_id, name, kind, webhook_url, payload_template, headers, secret, created_by)
+		VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)`,
+		t.ID, t.TenantID, t.Name, t.Kind, t.WebhookURL, t.PayloadTemplate, t.Headers, t.Secret, t.CreatedBy)
+	return err
+}
+
+func (s *Store) List(ctx context.Context) ([]Target, error) {
+	rows, err := s.pool.Query(ctx, `
+		SELECT id, tenant_id, name, kind, webhook_url, payload_template, headers, secret, created_by
+		FROM notification_targets ORDER BY created_at DESC`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var out []Target
+	for rows.Next() {
+		var t Target
+		if err := rows.Scan(&t.ID, &t.TenantID, &t.Name, &t.Kind, &t.WebhookURL, &t.PayloadTemplate, &t.Headers, &t.Secret, &t.CreatedBy); err != nil {
+			return nil, err
+		}
+		out = append(out, t)
+	}
+	return out, rows.Err()
+}
+
+func (s *Store) Get(ctx context.Context, id string) (*Target, error) {
+	var t Target
+	row := s.pool.QueryRow(ctx, `
+		SELECT id, tenant_id, name, kind, webhook_url, payload_template, headers, secret, created_by
+		FROM notification_targets WHERE id = $1`, id)
+	if err := row.Scan(&t.ID, &t.TenantID, &t.Name, &t.Kind, &t.WebhookURL, &t.PayloadTemplate, &t.Headers, &t.Secret, &t.CreatedBy); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, ErrNotFound
+		}
+		return nil, err
+	}
+	return &t, nil
+}
+
+func (s *Store) Delete(ctx context.Context, id string) error {
+	tag, err := s.pool.Exec(ctx, `DELETE FROM notification_targets WHERE id = $1`, id)
+	if err != nil {
+		return err
+	}
+	if tag.RowsAffected() == 0 {
+		return ErrNotFound
+	}
+	return nil
+}