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/src/lib/QueryBar.svelte

@@ -0,0 +1,93 @@
+<script lang="ts">
+	// Extracted from the root query page in Phase 3 so the dashboard panel
+	// editor and the alert rule editor can reuse the same input --
+	// deliberately just the input+run affordance, not results/history,
+	// which differ per consumer.
+	import type { Language } from '$lib/api';
+
+	let {
+		query = $bindable(''),
+		language = $bindable<Language>(''),
+		onRun,
+		loading = false,
+		placeholder = 'service=api | where status>=500 | stats count by host | sort -count'
+	}: {
+		query: string;
+		language: Language;
+		onRun: () => void;
+		loading?: boolean;
+		placeholder?: string;
+	} = $props();
+
+	// Client-side mirror of the backend's auto-detect heuristic
+	// (api/internal/querylang/planner.looksLikeSQL) -- purely a UI hint,
+	// the server does its own detection independently and is the
+	// authority on what actually runs.
+	function detectedLanguage(q: string): 'sql' | 'spl' {
+		return /^\s*select\b/i.test(q) ? 'sql' : 'spl';
+	}
+	let detected = $derived(detectedLanguage(query));
+	let effectiveLanguage = $derived(language === '' ? detected : language);
+
+	function onKeydown(e: KeyboardEvent) {
+		// Cmd/Ctrl+Enter runs the query -- textarea's own Enter key needs
+		// to stay newline-for-pipe-stage-formatting, so this isn't a bare
+		// Enter binding.
+		if ((e.metaKey || e.ctrlKey) && e.key === 'Enter') {
+			e.preventDefault();
+			onRun();
+		}
+	}
+</script>
+
+<div class="query-bar">
+	<textarea bind:value={query} onkeydown={onKeydown} rows="4" spellcheck="false" {placeholder}></textarea>
+	<div class="controls">
+		<label>
+			Language:
+			<select bind:value={language}>
+				<option value="">Auto ({detected})</option>
+				<option value="spl">Pipe syntax</option>
+				<option value="sql">SQL</option>
+			</select>
+		</label>
+		<span class="detected-badge" class:sql={effectiveLanguage === 'sql'}>
+			{effectiveLanguage === 'sql' ? 'SQL' : 'pipe syntax'}
+		</span>
+		<button onclick={onRun} disabled={loading || query.trim() === ''}>
+			{loading ? 'Running…' : 'Run query'}
+		</button>
+		<span class="hint">⌘/Ctrl+Enter to run</span>
+	</div>
+</div>
+
+<style>
+	.query-bar textarea {
+		width: 100%;
+		font-family: monospace;
+		font-size: 0.9rem;
+		box-sizing: border-box;
+	}
+	.controls {
+		margin-top: 0.5rem;
+		display: flex;
+		align-items: center;
+		gap: 0.75rem;
+		flex-wrap: wrap;
+	}
+	.detected-badge {
+		font-size: 0.75rem;
+		padding: 0.15rem 0.5rem;
+		border-radius: 1rem;
+		background: #eef;
+		color: #224;
+	}
+	.detected-badge.sql {
+		background: #fee;
+		color: #422;
+	}
+	.hint {
+		font-size: 0.8rem;
+		color: #777;
+	}
+</style>