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

tests/test_migration_0003.py

@@ -6,7 +6,6 @@ from backend.app.models import (
     MLSettings,
     TagAlias,
     TagAllowlist,
-    TagReferenceEmbedding,
     TagSuggestionRejection,
 )
 
@@ -16,7 +15,6 @@ def test_new_tables_registered():
         "tag_allowlist",
         "tag_suggestion_rejection",
         "tag_alias",
-        "tag_reference_embedding",
         "ml_settings",
     }
     assert expected.issubset(Base.metadata.tables.keys())
@@ -42,12 +40,6 @@ def test_ml_settings_singleton_constraint():
     assert "ck_ml_settings_singleton" in names
 
 
-def test_tag_reference_embedding_has_vector():
-    cols = {c.name for c in TagReferenceEmbedding.__table__.columns}
-    assert "embedding" in cols
-    assert "reference_count" in cols
-
-
 def test_tag_allowlist_confidence_check():
     names = {c.name for c in TagAllowlist.__table__.constraints}
     assert "ck_tag_allowlist_confidence_range" in names