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

backend/app/api/ml_admin.py

@@ -1,4 +1,4 @@
-"""ML admin API: settings, backfill trigger, centroid recompute trigger."""
+"""ML admin API: settings + backfill trigger."""
 
 from quart import Blueprint, jsonify, request
 
@@ -11,8 +11,6 @@ ml_admin_bp = Blueprint("ml_admin", __name__, url_prefix="/api/ml")
 _EDITABLE = (
     "suggestion_threshold_character",
     "suggestion_threshold_general",
-    "centroid_similarity_threshold",
-    "min_reference_images",
     "tagger_store_floor",
     "video_frame_interval_seconds",
     "video_max_frames",
@@ -41,8 +39,6 @@ async def get_settings():
         {
             "suggestion_threshold_character": s.suggestion_threshold_character,
             "suggestion_threshold_general": s.suggestion_threshold_general,
-            "centroid_similarity_threshold": s.centroid_similarity_threshold,
-            "min_reference_images": s.min_reference_images,
             "tagger_store_floor": s.tagger_store_floor,
             "video_frame_interval_seconds": s.video_frame_interval_seconds,
             "video_max_frames": s.video_max_frames,
@@ -142,11 +138,3 @@ async def trigger_backfill():
 
     r = backfill.delay()
     return jsonify({"celery_task_id": r.id}), 202
-
-
-@ml_admin_bp.route("/recompute-centroids", methods=["POST"])
-async def trigger_recompute():
-    from ..tasks.ml import recompute_centroids
-
-    r = recompute_centroids.delay()
-    return jsonify({"celery_task_id": r.id}), 202

backend/app/api/tags.py

@@ -304,12 +304,6 @@ async def merge_tag(source_id: int):
         from ..tasks.ml import apply_allowlist_tags
 
         apply_allowlist_tags.delay(tag_id=result.target_id)
-    # Tag merge invalidates the target's centroid (the merged-in source
-    # tag's images now contribute to it). Daily list_drifted catches it
-    # within 24h, but eager recompute closes the suggestion-quality dip
-    # in the meantime. Audit 2026-06-02.
-    from ..tasks.ml import recompute_centroid
-    recompute_centroid.delay(result.target_id)
     return jsonify(
         {
             "target": {