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/api/admin/analysis.rs

@@ -44,6 +44,7 @@ pub fn routes() -> Router<AppState> {
         .route("/admin/analysis/pages/:id", get(page_detail))
         .route("/admin/analysis/history", get(list_history))
         .route("/admin/analysis/metrics", get(metrics))
+        .route("/admin/analysis/metrics/series", get(metrics_series))
         .route("/admin/analysis/status/stream", get(stream_status))
 }
 
@@ -215,6 +216,46 @@ pub(super) fn window_since(days: i64) -> Option<chrono::DateTime<chrono::Utc>> {
     }
 }
 
+// --- shared trend-series plumbing (crawler + analysis charts) ---------------
+
+/// Query params for the bucketed trend-series endpoints. `bucket` is
+/// optional; when absent it's derived from `days`.
+#[derive(Debug, Deserialize, Default)]
+pub struct SeriesParams {
+    #[serde(default)]
+    pub days: i64,
+    #[serde(default)]
+    pub bucket: Option<String>,
+}
+
+/// Envelope for a trend series.
+#[derive(Debug, Serialize)]
+pub struct SeriesResponse<T> {
+    pub buckets: Vec<T>,
+}
+
+/// Resolve the `date_trunc` granularity: an explicit `bucket` (validated to
+/// `hour`|`day`, else 400) wins; otherwise short windows bucket by hour and
+/// longer ones by day so the point count stays chart-friendly.
+pub(super) fn resolve_bucket(
+    days: i64,
+    explicit: Option<&str>,
+) -> Result<crate::repo::crawl_metrics::Bucket, AppError> {
+    use crate::repo::crawl_metrics::Bucket;
+    match explicit {
+        Some("hour") => Ok(Bucket::Hour),
+        Some("day") => Ok(Bucket::Day),
+        Some(other) => Err(AppError::InvalidInput(format!(
+            "invalid bucket '{other}' (expected 'hour' or 'day')"
+        ))),
+        None => Ok(if days > 0 && days <= 2 {
+            Bucket::Hour
+        } else {
+            Bucket::Day
+        }),
+    }
+}
+
 async fn metrics(
     State(state): State<AppState>,
     _admin: RequireAdmin,
@@ -225,6 +266,17 @@ async fn metrics(
     Ok(Json(m))
 }
 
+async fn metrics_series(
+    State(state): State<AppState>,
+    _admin: RequireAdmin,
+    Query(params): Query<SeriesParams>,
+) -> AppResult<Json<SeriesResponse<crate::domain::crawl_metrics::MetricsBucket>>> {
+    let bucket = resolve_bucket(params.days, params.bucket.as_deref())?;
+    let buckets =
+        repo::page_analysis::analysis_series(&state.db, bucket, window_since(params.days)).await?;
+    Ok(Json(SeriesResponse { buckets }))
+}
+
 #[derive(Debug, Deserialize)]
 pub struct ReenqueueBody {
     /// Skip pages that already have a `done` analysis row. Defaults to