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>

alembic/versions/0044_ml_settings_tagger_store_floor.py

@@ -0,0 +1,37 @@
+"""ml_settings.tagger_store_floor
+
+The ingest confidence floor below which tagger predictions are not stored,
+promoted from the TAGGER_STORE_FLOOR env var to a DB-backed, UI-tunable
+setting. Default 0.70 (was an env default of 0.05): the suggestion path
+already filters at 0.70 and the centroid/learned path covers low-confidence
+preferred tags, so the sub-0.70 tail was redundant weight — it had grown
+image_record's TOAST to ~100 GB. See plan-task #764.
+
+Revision ID: 0044
+Revises: 0043
+Create Date: 2026-06-10
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0044"
+down_revision: Union[str, None] = "0043"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "tagger_store_floor", sa.Float(),
+            nullable=False, server_default="0.7",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "tagger_store_floor")