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/nginx.conf

@@ -4,6 +4,15 @@ server {
     index index.html;
 
     location / {
-        try_files $uri $uri/ /index.html;
+        # adapter-static writes prerendered routes as flat <route>.html
+        # files (e.g. /dashboards -> dashboards.html, confirmed by
+        # actually inspecting the build output), not <route>/index.html --
+        # $uri.html has to be in this chain or a request for exactly
+        # "/dashboards" falls straight through to the SPA fallback and
+        # skips the prerendered page it should be serving. 200.html (not
+        # index.html) is the fallback shell for genuinely dynamic routes
+        # (dashboards/[id] etc.) -- named differently so it doesn't
+        # collide with "/" -> index.html, which really is prerendered.
+        try_files $uri $uri.html $uri/ /200.html;
     }
 }