feat(settings): tag-eval admin card — trigger + persisted report (survives nav)

Frontend for #1130. A maintenance tile in Settings → Tagging:
- Editable concept list + "Run eval" → POST /api/tag-eval (one running at a time).
- Rehydrates on mount via the persisted run (getRun by latest id) and polls while
  running — so the report SURVIVES navigation (operator-flagged); the task runs
  backend-side regardless and the card reconnects to its row.
- Renders the saved report: per-concept head-vs-centroid metrics table (AP/F1/
  precision/recall) with Δ AP, the learning curve (AP @ N positives), and
  thumbnail galleries (head-would-suggest / head-doubts-positive) for eyeballing.

Backend: _examples now stores thumbnail_urls (not just ids) so the report is a
self-contained artifact that renders without per-id lookups on reload.

No new top-level surface — slots into the existing maintenance area.

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

frontend/src/components/settings/MaintenancePanel.vue

@@ -28,6 +28,7 @@
         <MLThresholdSliders />
         <AllowlistTable />
         <AliasTable />
+        <TagEvalCard />
       </div>
     </section>
 
@@ -53,6 +54,7 @@ import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
 import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'
+import TagEvalCard from './TagEvalCard.vue'
 import BackupCard from './BackupCard.vue'
 import { useSystemActivityStore } from '../../stores/systemActivity.js'