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>

backend/app/models/__init__.py

@@ -9,9 +9,14 @@ from .image_record import ImageRecord
 from .import_batch import ImportBatch
 from .import_settings import ImportSettings
 from .import_task import ImportTask
+from .ml_settings import MLSettings
 from .post import Post
 from .source import Source
 from .tag import Tag, TagKind, image_tag
+from .tag_alias import TagAlias
+from .tag_allowlist import TagAllowlist
+from .tag_reference_embedding import TagReferenceEmbedding
+from .tag_suggestion_rejection import TagSuggestionRejection
 
 __all__ = [
     "Base",
@@ -28,4 +33,9 @@ __all__ = [
     "ImportBatch",
     "ImportTask",
     "ImportSettings",
+    "MLSettings",
+    "TagAlias",
+    "TagAllowlist",
+    "TagReferenceEmbedding",
+    "TagSuggestionRejection",
 ]