refactor(ml): remove the dead per-tag centroid subsystem (#1189)

The v2 pivot replaced per-tag SigLIP centroids with learned heads + CCIP.
Centroids were still recomputed (on every tag merge + a daily beat) but NOTHING
read them — suggestions come from heads+CCIP and apply_allowlist_tags applies
via Camie predictions, not centroids. Pure dead wiring; remove it.

Removed: CentroidService, recompute_centroid/recompute_centroids tasks, the
daily beat, POST /api/ml/recompute-centroids, the recompute-on-merge trigger,
the tag_reference_embedding table + model, the centroid_similarity_threshold +
min_reference_images settings (migration 0066), the CentroidRecomputeCard +
its store action + MaintenancePanel tile, and the centroid slider in
MLThresholdSliders. _keep_as_alias drops its vestigial has-centroid branch (the
allowlist branch already covers "could re-emit"); tag merge no longer clears a
table that no longer exists.

NOT touched (still live, parallel to heads): the Camie tagger, ImagePrediction,
and the allowlist bulk-apply — accepting a suggestion still allowlists + applies
it across the library. The tag-eval "centroid" baseline metric is unrelated
(in-memory) and stays. (image_record.centroid_scores JSON column also remains —
separate legacy field, its own micro-cleanup.)

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

frontend/src/components/settings/MaintenancePanel.vue

@@ -1,9 +1,8 @@
 <template>
   <div class="fc-maint">
     <p class="fc-muted text-body-2 mb-5">
-      One-off backfills, tagging config and storage tools. The ML backfill and
-      centroid recompute also run nightly; the allowlist auto-applies accepted
-      tags. Click a tile to open it.
+      One-off backfills, tagging config and storage tools. The ML backfill runs
+      nightly; the allowlist auto-applies accepted tags. Click a tile to open it.
     </p>
 
     <section class="fc-section">
@@ -11,7 +10,6 @@
       <p class="fc-section__hint">Re-run tagging, thumbnails, extraction and DB upkeep.</p>
       <div class="fc-tile-grid">
         <MLBackfillCard />
-        <CentroidRecomputeCard />
         <ThumbnailBackfillCard />
         <ArchiveReextractCard />
         <MissingFileRepairCard />
@@ -48,7 +46,6 @@
 import { onMounted, onUnmounted } from 'vue'
 
 import MLBackfillCard from './MLBackfillCard.vue'
-import CentroidRecomputeCard from './CentroidRecomputeCard.vue'
 import ThumbnailBackfillCard from './ThumbnailBackfillCard.vue'
 import ArchiveReextractCard from './ArchiveReextractCard.vue'
 import MissingFileRepairCard from './MissingFileRepairCard.vue'