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>

backend/src/domain/crawl_metrics.rs

@@ -11,6 +11,20 @@ use serde::Serialize;
 use sqlx::FromRow;
 use uuid::Uuid;
 
+/// One time bucket of a metrics series — count, ok/failed split, and mean
+/// duration over the bucket. Shared by the crawl-ops and analysis trend
+/// charts (both are `{t, n, ok, failed, avg_ms}` over a `date_trunc` window).
+/// Empty intervals are simply absent; the client fills the continuous axis.
+#[derive(Debug, Clone, Serialize, FromRow)]
+pub struct MetricsBucket {
+    /// Bucket start (`date_trunc(unit, finished_at|analyzed_at)`).
+    pub t: DateTime<Utc>,
+    pub n: i64,
+    pub ok: i64,
+    pub failed: i64,
+    pub avg_ms: Option<f64>,
+}
+
 /// Per-operation-type average roll-up over a time window.
 #[derive(Debug, Clone, Serialize, FromRow)]
 pub struct OpSummary {