feat(fc2b): schema migration 0003 — ML pipeline tables

Renames image_record.wd14_* -> tagger_* (we're on Camie now, not WD14).
Adds tag_allowlist (auto-apply opt-in, per-tag confidence),
tag_suggestion_rejection (per-image dismissals), tag_alias (composite
(string, category) -> canonical tag, resolved at read time),
tag_reference_embedding (per-tag SigLIP centroids), and the ml_settings
singleton (per-category + centroid thresholds, model version pins).

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

tests/test_migration_0003.py

@@ -0,0 +1,56 @@
+"""Smoke test for migration 0003: model classes import, schema shape correct."""
+
+from backend.app.models import (
+    Base,
+    ImageRecord,
+    MLSettings,
+    TagAlias,
+    TagAllowlist,
+    TagReferenceEmbedding,
+    TagSuggestionRejection,
+)
+
+
+def test_new_tables_registered():
+    expected = {
+        "tag_allowlist",
+        "tag_suggestion_rejection",
+        "tag_alias",
+        "tag_reference_embedding",
+        "ml_settings",
+    }
+    assert expected.issubset(Base.metadata.tables.keys())
+
+
+def test_image_record_columns_renamed():
+    cols = {c.name for c in ImageRecord.__table__.columns}
+    assert "tagger_predictions" in cols
+    assert "tagger_model_version" in cols
+    assert "wd14_predictions" not in cols
+    assert "wd14_model_version" not in cols
+
+
+def test_tag_alias_composite_pk():
+    pk_cols = {c.name for c in TagAlias.__table__.primary_key.columns}
+    assert pk_cols == {"alias_string", "alias_category"}
+
+
+def test_ml_settings_singleton_constraint():
+    names = {c.name for c in MLSettings.__table__.constraints}
+    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
+
+
+def test_tag_suggestion_rejection_pk():
+    pk_cols = {c.name for c in TagSuggestionRejection.__table__.primary_key.columns}
+    assert pk_cols == {"image_record_id", "tag_id"}