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.

metadata/migrate.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Applies migrations/*.sql to Postgres in filename order, tracking what's
+# already been applied in a schema_migrations table -- same shape as
+# /storage/migrate.sh, adapted for psql instead of curl since Postgres
+# supports real transactions per file (kept to one DDL object per file
+# anyway, for repo-wide consistency of what a migration "version" means).
+set -euo pipefail
+
+POSTGRES_HOST="${POSTGRES_HOST:-localhost}"
+POSTGRES_PORT="${POSTGRES_PORT:-5432}"
+POSTGRES_USER="${POSTGRES_USER:-sentry}"
+POSTGRES_PASSWORD="${POSTGRES_PASSWORD:-}"
+POSTGRES_DATABASE="${POSTGRES_DATABASE:-sentry_metadata}"
+
+export PGPASSWORD="$POSTGRES_PASSWORD"
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+MIGRATIONS_DIR="${SCRIPT_DIR}/migrations"
+
+psql_exec() {
+    psql -v ON_ERROR_STOP=1 -X -q -h "$POSTGRES_HOST" -p "$POSTGRES_PORT" -U "$POSTGRES_USER" -d "$POSTGRES_DATABASE" "$@"
+}
+
+echo "Ensuring schema_migrations table exists..."
+psql_exec -c "CREATE TABLE IF NOT EXISTS schema_migrations (version TEXT PRIMARY KEY, applied_at TIMESTAMPTZ NOT NULL DEFAULT now())"
+
+applied="$(psql_exec -t -A -c "SELECT version FROM schema_migrations")"
+
+shopt -s nullglob
+for file in "${MIGRATIONS_DIR}"/*.sql; do
+    version="$(basename "$file")"
+    if grep -qx "$version" <<< "$applied"; then
+        echo "skip  ${version} (already applied)"
+        continue
+    fi
+    echo "apply ${version}"
+    psql_exec -f "$file"
+    psql_exec -c "INSERT INTO schema_migrations (version) VALUES ('${version}')"
+done
+
+echo "Migrations complete."