feat(metrics): per-app observability dashboard over execution_logs (A2)

Aggregates the existing execution_logs table (no hot-path instrumentation):
ExecutionLogRepository::summarize_for_app returns counts, error rate, latency
avg/p50/p95 (percentile_cont), by-status/by-source breakdowns, and an hourly
series over a trailing window (clamped 1h..90d). New metrics_api router at
GET /api/v1/admin/apps/{id}/metrics?window= (AppLogRead), and a dashboard
Metrics tab (summary cards + an inline-SVG per-hour bar chart, no JS dep).

Pinned by a manager-core test asserting the exact rollup (percentiles included)
and the empty-app zero case.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

dashboard/src/lib/AppTabBar.svelte

@@ -39,6 +39,7 @@
 			href: `${base}/apps/${slug}/workflows`,
 			adminOnly: true
 		},
+		{ id: 'metrics', label: 'Metrics', href: `${base}/apps/${slug}/metrics`, adminOnly: true },
 		{
 			id: 'dead-letters',
 			label: 'Dead letters',

dashboard/src/lib/api.ts

@@ -218,6 +218,33 @@ export interface ExecutionLog {
 	created_at: string;
 }
 
+/** A2: one `(key, count)` row of a metrics breakdown. */
+export interface MetricsCount {
+	key: string;
+	count: number;
+}
+
+/** A2: one hour-bucket of the metrics time series. */
+export interface MetricsBucket {
+	ts: string;
+	total: number;
+	errors: number;
+}
+
+/** A2: the observability rollup for one app over a trailing window. */
+export interface MetricsSummary {
+	window_hours: number;
+	total: number;
+	errors: number;
+	error_rate: number;
+	avg_duration_ms: number;
+	p50_duration_ms: number;
+	p95_duration_ms: number;
+	by_status: MetricsCount[];
+	by_source: MetricsCount[];
+	series: MetricsBucket[];
+}
+
 export interface CreateScriptInput {
 	app_id: string;
 	name: string;
@@ -1169,6 +1196,14 @@ export const api = {
 			)
 	},
 
+	metrics: {
+		/** A2: aggregate execution metrics for an app over the trailing window (hours). */
+		summary: (idOrSlug: string, windowHours = 24) =>
+			adminRequest<MetricsSummary>(
+				`/api/v1/admin/apps/${encodeURIComponent(idOrSlug)}/metrics?window=${windowHours}`
+			)
+	},
+
 	deadLetters: {
 		count: (idOrSlug: string) =>
 			adminRequest<{ unresolved: number }>(