feat: chart-led Security overview with sortable Images and History tables (#1364)

* feat: chart-led Security overview with sortable Images and History tables

Refine the Security page around the existing design system and add the
data the dashboard needs.

- Overview leads with four charts (30-day risk trend, severity donut, top
  exposed images, findings by type); the signal-rail counts become a
  secondary summary, and the scanner and deploy-enforcement posture follow.
- Images becomes a recessed table with search, a severity filter, sortable
  columns, a last-scan column, and inline scan actions; the findings cell is
  clickable into the scan sheet, and the per-row cursor tooltip is dropped
  where the columns already carry that information.
- Policies puts deploy-enforcement first, collapses the policy packs into an
  accordion, and uses the standard primary button for Add policy.
- Suppressions and acknowledgements move their titles and Add buttons outside
  the cards, matching the Fleet tab layout.
- History switches from the detail sheet to an inline table (search, sortable
  columns, two-scan compare, pagination); the now-unreachable scan-history
  overlay is removed.
- Add GET /api/security/overview/trend, a node-scoped daily critical/high
  rollup backing the risk-trend chart.
- Extract the shared image-scan hook and the severity classifier, and harden
  the overview data fetch so a malformed non-critical response can never read
  as a clean security state.

* fix: treat malformed Security responses as errors, not empty or clean states

Address an independent review of the data-fetch paths so a 200 with an
unexpected shape can never read as a benign "no findings" view.

- SecurityView: validate that the image-summaries body is a scan-summary map; an
  unexpected shape now sets the error state instead of an empty map. Isolate the
  trend fetch in its own self-catching promise so a transport failure on the
  non-critical chart can no longer poison the overview or summaries error state.
- useImageScan: only a "completed" poll counts as success (a malformed or unknown
  status now throws), and a failed post-scan summaries refresh is logged instead
  of silently dropped.
- HistoryTab: a 200 whose body lacks an items array is treated as an error, not
  an empty "no completed scans" list.

frontend/src/lib/severityStyles.ts

@@ -1,4 +1,17 @@
-import type { VulnSeverity } from '@/types/security';
+import type { ScanSummary, VulnSeverity } from '@/types/security';
+
+export type SeverityKey = VulnSeverity | 'CLEAN' | 'FINDINGS';
+
+/**
+ * The display "key" for a scan summary: its highest vulnerability severity, or
+ * `FINDINGS` when the scan has only secrets/misconfigurations (stored
+ * highest_severity is derived from CVEs alone), or `CLEAN` when nothing was
+ * found. Shared so the badge, sorting, and filtering all classify identically.
+ */
+export function getSeverityKey(summary: ScanSummary): SeverityKey {
+  const hasNonVulnFindings = (summary.secret_count ?? 0) > 0 || (summary.misconfig_count ?? 0) > 0;
+  return summary.highest_severity ?? (hasNonVulnFindings ? 'FINDINGS' : 'CLEAN');
+}
 
 export const SEVERITY_ROW_TINT: Record<VulnSeverity, string> = {
   CRITICAL: 'bg-destructive/10 border-l-[3px] border-destructive/70',
@@ -14,7 +27,7 @@ export const SEVERITY_ROW_TINT: Record<VulnSeverity, string> = {
  * state for a scan that has secrets or misconfigurations but zero CVEs (the
  * stored highest_severity is derived from vulnerabilities only).
  */
-export const SEVERITY_BADGE_CLASSES: Record<VulnSeverity | 'CLEAN' | 'FINDINGS', string> = {
+export const SEVERITY_BADGE_CLASSES: Record<SeverityKey, string> = {
   CRITICAL: 'border-destructive/25 bg-destructive/8 text-destructive',
   HIGH: 'border-warning/25 bg-warning/8 text-warning',
   MEDIUM: 'border-warning/25 bg-warning/8 text-warning',
@@ -25,7 +38,7 @@ export const SEVERITY_BADGE_CLASSES: Record<VulnSeverity | 'CLEAN' | 'FINDINGS',
 };
 
 /** Leading state-dot color for a severity pill. */
-export const SEVERITY_DOT_CLASSES: Record<VulnSeverity | 'CLEAN' | 'FINDINGS', string> = {
+export const SEVERITY_DOT_CLASSES: Record<SeverityKey, string> = {
   CRITICAL: 'bg-destructive',
   HIGH: 'bg-warning',
   MEDIUM: 'bg-warning',