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.

web/vite.config.ts

@@ -9,7 +9,16 @@ export default defineConfig({
 				// Force runes mode for the project, except for libraries. Can be removed in svelte 6.
 				runes: ({ filename }) => filename.split(/[/\\]/).includes('node_modules') ? undefined : true
 			},
-			adapter: adapter()
+			// fallback: dashboards/[id] and alerts/[id] are dynamic routes
+			// whose params (dashboard/rule IDs) don't exist at build time, so
+			// they can't be prerendered like the root/list pages. Named
+			// 200.html, not index.html -- the root route ("/") is itself
+			// prerendered to index.html, and a same-named fallback silently
+			// overwrites it (confirmed by actually running the build: "Using
+			// index.html" produced the warning "Overwriting build/index.html
+			// with fallback page"). nginx.conf's try_files points at
+			// /200.html to match.
+			adapter: adapter({ fallback: '200.html' })
 		})
 	]
 });