chore(ml): suggestion_threshold default 0.50 → 0.70

Operator-flagged 2026-06-02 — the 0.50 default (set on 2026-06-01)
surfaces too many low-confidence picks in the modal's Suggestions
rail. 0.70 keeps the rail signal-rich while still showing more than
the original 0.95 (which hid almost everything).

Alembic 0033 updates the singleton row conditionally — only rows
still at the old 0.50 default flip to 0.70. Operators who tuned to
some other value via Settings → ML keep their pick.

Settings UI already exposes both sliders (MLThresholdSliders.vue),
so further tuning continues to work without a deploy.

backend/app/models/ml_settings.py

@@ -16,13 +16,14 @@ class MLSettings(Base):
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     suggestion_threshold_character: Mapped[float] = mapped_column(
-        Float, nullable=False, default=0.50
+        Float, nullable=False, default=0.70
     )
-    # Default lowered 0.95 → 0.50 on 2026-06-01 — operator-flagged that
-    # 0.95 hid most general suggestions. Operator-tunable via Settings →
-    # ML if too noisy.
+    # Default raised 0.50 → 0.70 on 2026-06-02 — operator-flagged 0.50
+    # surfaced too many low-confidence picks; 0.70 keeps the rail
+    # signal-rich while still surfacing more than the original 0.95
+    # which hid almost everything. Operator-tunable via Settings → ML.
     suggestion_threshold_general: Mapped[float] = mapped_column(
-        Float, nullable=False, default=0.50
+        Float, nullable=False, default=0.70
     )
     centroid_similarity_threshold: Mapped[float] = mapped_column(
         Float, nullable=False, default=0.55