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/config/config.go

@@ -12,6 +12,7 @@ import (
 type Config struct {
 	HTTPListenAddr    string
 	ClickHouse        ClickHouseConfig
+	Postgres          PostgresConfig
 	SearchGRPCAddr    string
 	QueryTimeout      time.Duration
 	CORSAllowedOrigin string
@@ -24,6 +25,16 @@ type ClickHouseConfig struct {
 	Password string
 }
 
+// PostgresConfig is the control-plane metadata store (dashboards, panels
+// -- see /docs/phase-3-dashboard-design.md), distinct from ClickHouse
+// which remains log-data-only.
+type PostgresConfig struct {
+	Addr     string
+	Database string
+	Username string
+	Password string
+}
+
 func Load() (Config, error) {
 	cfg := Config{
 		HTTPListenAddr: getenv("HTTP_LISTEN_ADDR", ":8080"),
@@ -33,6 +44,12 @@ func Load() (Config, error) {
 			Username: getenv("CLICKHOUSE_USERNAME", "default"),
 			Password: getenv("CLICKHOUSE_PASSWORD", ""),
 		},
+		Postgres: PostgresConfig{
+			Addr:     getenv("POSTGRES_ADDR", "localhost:5432"),
+			Database: getenv("POSTGRES_DATABASE", "sentry_metadata"),
+			Username: getenv("POSTGRES_USERNAME", "sentry"),
+			Password: getenv("POSTGRES_PASSWORD", ""),
+		},
 		// Search service's gRPC address (see /search) -- default matches
 		// /search's own default GRPC_LISTEN_ADDR.
 		SearchGRPCAddr: getenv("SEARCH_GRPC_ADDR", "localhost:50052"),