feat(admin): time-series trend charts on the metrics tabs (0.87.0)

Turn the dormant timing data in crawl_metrics / page_analysis into "is it
healthy over time" views. New bucketed series queries (GROUP BY date_trunc,
hour|day via a closed Bucket enum so the unit can't be attacker-controlled)
behind GET /v1/admin/{crawler,analysis}/metrics/series, with a shared
SeriesParams/resolve_bucket helper (bad bucket → 400) and migration 0030
indexing page_analysis(analyzed_at) for the analysis scan.

Frontend: a dependency-free SVG TrendChart (line+area, null buckets render as
gaps, empty-state, role=img) embedded above the per-op tables in Crawler and
Analysis → Metrics, driven by each panel's existing window selector with
AbortController-cancelled fetches. A buildSeries() util fills the continuous
bucket axis (throughput 0 for empty buckets, success/duration null) — unit
tested alongside the chart and the series API client.

Closes the Phase-1 observability set (audit log · health checks · trends).

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

frontend/src/lib/api/admin.test.ts

@@ -45,7 +45,9 @@ import {
     getAnalysisMetrics,
     listAuditLog,
     getHealth,
-    updateHealthThresholds
+    updateHealthThresholds,
+    getCrawlerMetricsSeries,
+    getAnalysisMetricsSeries
 } from './admin';
 
 function ok(body: unknown, status = 200): Response {
@@ -532,6 +534,20 @@ describe('admin crawler api client', () => {
         }
     };
 
+    it('getCrawlerMetricsSeries GETs the crawler series with days', async () => {
+        fetchSpy.mockResolvedValueOnce(ok({ buckets: [] }));
+        await getCrawlerMetricsSeries(7);
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/crawler\/metrics\/series\?days=7$/);
+    });
+
+    it('getAnalysisMetricsSeries omits days=0 (all-time)', async () => {
+        fetchSpy.mockResolvedValueOnce(ok({ buckets: [] }));
+        await getAnalysisMetricsSeries(0);
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/admin\/analysis\/metrics\/series$/);
+    });
+
     it('getHealth GETs /v1/admin/health', async () => {
         fetchSpy.mockResolvedValueOnce(ok(healthFixture));
         const r = await getHealth();