feat(heads): admin card to train + inspect concept heads (#114 B)

The UI for the heads subsystem: Settings → Tagging → "Concept heads". Shows
head count, auto-apply-ready count, and last-trained; a Train/Retrain button
(one run at a time, polls while running, surfaces a failed run's error); an
empty state guiding the operator to tag first; and a per-concept table (name,
category, +tags, AP, P, R, auto-apply ⚡) sorted strongest-first so weak/under-
tagged concepts are obvious. Rehydrates status from GET /api/heads on mount so
it survives navigation. Pulls head_min_positives from ML settings for copy.

Slice C (swap the rail's suggestions to heads, remove Camie + centroid) is next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/components/settings/MaintenancePanel.vue

@@ -26,6 +26,7 @@
       </p>
       <div class="fc-tile-stack">
         <MLThresholdSliders />
+        <HeadsCard />
         <AllowlistTable />
         <AliasTable />
         <TagEvalCard />
@@ -52,6 +53,7 @@ import ArchiveReextractCard from './ArchiveReextractCard.vue'
 import MissingFileRepairCard from './MissingFileRepairCard.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
+import HeadsCard from './HeadsCard.vue'
 import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'
 import TagEvalCard from './TagEvalCard.vue'