feat(ml): DB-backed tagger_store_floor (default 0.70), the ingest confidence floor

Promotes the prediction store-floor from the TAGGER_STORE_FLOOR env (default
0.05) to a DB-backed, Settings-UI-tunable ml_settings column (default 0.70).
Storing every tag down to 0.05 from a ~10k-tag tagger is what grew
image_record's TOAST to ~100 GB; the suggestion path already filters at 0.70
and the centroid/learned path covers lower-confidence preferred tags, so the
sub-0.70 tail is redundant. Foundation for plan-task #764 (backfill + reclaim
land next; this only changes the write gate for NEW imports).

- ml_settings.tagger_store_floor (migration 0044, default 0.70)
- tagger.Tagger.infer(store_floor=...); ml task passes settings.tagger_store_floor
- ML admin GET/PATCH expose it; PATCH rejects a category suggestion threshold
  below the floor (nothing below the floor is stored, so the gap surfaces
  nothing) — server backstop for the UI slider clamp
- Settings → ML: store-floor slider + caption; category sliders min-bound to it

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/models/ml_settings.py

@@ -28,6 +28,15 @@ class MLSettings(Base):
     centroid_similarity_threshold: Mapped[float] = mapped_column(
         Float, nullable=False, default=0.55
     )
+    # Ingest floor: tagger predictions below this confidence are not stored
+    # (tagger.Tagger.infer). Default 0.70 — the suggestion path already
+    # filters at 0.70 and the centroid/learned path covers low-confidence
+    # preferred tags, so the sub-0.70 tail is redundant weight (it had
+    # bloated image_record's TOAST to ~100 GB; plan-task #764). Operator-
+    # tunable via Settings → ML; must stay ≤ the suggestion thresholds.
+    tagger_store_floor: Mapped[float] = mapped_column(
+        Float, nullable=False, default=0.70
+    )
     min_reference_images: Mapped[int] = mapped_column(
         Integer, nullable=False, default=5
     )