feat(ml): clamp allowlist min_confidence to the tagger store floor

Consumer #4 of the store-floor change (#764). An allowlist tag can't
auto-apply more permissively than the ingest floor — predictions below
tagger_store_floor aren't stored, so a lower min_confidence behaves
identically to the floor. update_threshold now clamps to max(value, floor);
the AllowlistTable confidence input min-binds to the live floor and clamps
on edit. Keeps the stored threshold honest about actual apply behavior.

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

backend/app/services/ml/allowlist.py

@@ -9,7 +9,7 @@ from sqlalchemy import delete, select
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from ...models import Tag, TagAllowlist, TagSuggestionRejection
+from ...models import MLSettings, Tag, TagAllowlist, TagSuggestionRejection
 from ...models.tag import image_tag
 from .aliases import AliasService
 
@@ -91,12 +91,25 @@ class AllowlistService:
         )
         await self.dismiss(image_id, tag_id)
 
+    async def _store_floor(self) -> float:
+        return (
+            await self.session.execute(
+                select(MLSettings.tagger_store_floor).where(MLSettings.id == 1)
+            )
+        ).scalar_one()
+
     async def update_threshold(
         self, tag_id: int, min_confidence: float
     ) -> None:
         row = await self.session.get(TagAllowlist, tag_id)
         if row is not None:
-            row.min_confidence = min_confidence
+            # An allowlist tag can't auto-apply more permissively than the
+            # ingest store floor — predictions below tagger_store_floor aren't
+            # stored, so a lower min_confidence would behave identically to the
+            # floor. Clamp so the stored threshold matches actual behavior
+            # (#764).
+            floor = await self._store_floor()
+            row.min_confidence = max(min_confidence, floor)
 
     async def remove(self, tag_id: int) -> None:
         await self.session.execute(