fix(fc2b): bare CHECK constraint names (naming-convention double-prefix)

Base.metadata's convention applies ck_%(table_name)s_%(constraint_name)s.
ml_settings and tag_allowlist passed already-prefixed names
(ck_ml_settings_singleton / ck_tag_allowlist_confidence_range), so the
ORM-side names came out doubled (ck_ml_settings_ck_ml_settings_singleton
etc.) and the migration-0003 smoke tests failed.

Same class of bug fixed in FC-2a for ImportSettings — should have applied
that lesson here. Bare names ('singleton', 'confidence_range') let the
convention produce the final names that match migration 0003's literal
DDL. Migration unchanged; only the model __table_args__.

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

backend/app/models/ml_settings.py

@@ -10,7 +10,9 @@ from .base import Base
 
 class MLSettings(Base):
     __tablename__ = "ml_settings"
-    __table_args__ = (CheckConstraint("id = 1", name="ck_ml_settings_singleton"),)
+    # Bare name — Base.metadata's naming convention prepends ck_<table>_,
+    # producing the final ck_ml_settings_singleton (matches migration 0003).
+    __table_args__ = (CheckConstraint("id = 1", name="singleton"),)
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     suggestion_threshold_artist: Mapped[float] = mapped_column(

backend/app/models/tag_allowlist.py

@@ -10,10 +10,12 @@ from .base import Base
 
 class TagAllowlist(Base):
     __tablename__ = "tag_allowlist"
+    # Bare name — Base.metadata's naming convention prepends ck_<table>_,
+    # producing the final ck_tag_allowlist_confidence_range (matches migration 0003).
     __table_args__ = (
         CheckConstraint(
             "min_confidence > 0 AND min_confidence <= 1",
-            name="ck_tag_allowlist_confidence_range",
+            name="confidence_range",
         ),
     )