feat(fc2b): Settings Maintenance tab

MaintenancePanel hosts: backfill + centroid recompute trigger cards,
the five suggestion-threshold sliders (autosave on slider release),
the allowlist table (inline editable min_confidence, delete), and the
alias table (mapping display, delete). Wired as a third Settings tab,
ML settings loaded lazily when the tab opens.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/MaintenancePanel.vue

@@ -0,0 +1,32 @@
+<template>
+  <div class="fc-maint">
+    <p class="text-body-2 mb-4">
+      Machine-assisted tagging controls. Backfill and centroid recompute run
+      nightly automatically; the allowlist auto-applies accepted tags to new
+      and existing images.
+    </p>
+    <div class="fc-maint__grid">
+      <MLBackfillCard />
+      <CentroidRecomputeCard />
+    </div>
+    <MLThresholdSliders class="mt-4" />
+    <AllowlistTable class="mt-4" />
+    <AliasTable class="mt-4" />
+  </div>
+</template>
+
+<script setup>
+import MLBackfillCard from './MLBackfillCard.vue'
+import CentroidRecomputeCard from './CentroidRecomputeCard.vue'
+import MLThresholdSliders from './MLThresholdSliders.vue'
+import AllowlistTable from './AllowlistTable.vue'
+import AliasTable from './AliasTable.vue'
+</script>
+
+<style scoped>
+.fc-maint__grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
+  gap: 16px;
+}
+</style>