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>

frontend/src/components/settings/MLThresholdSliders.vue

@@ -6,9 +6,28 @@
         <v-col cols="12">
           <v-slider
             v-model="local[f.key]" :label="f.label"
+            :min="f.floorMin ? local.tagger_store_floor : 0" max="1" step="0.05"
+            thumb-label hide-details
+            color="accent" @end="save"
+          />
+        </v-col>
+      </v-row>
+
+      <v-divider class="my-4" />
+
+      <v-row>
+        <v-col cols="12">
+          <v-slider
+            v-model="local.tagger_store_floor" label="Tagger store floor"
             min="0" max="1" step="0.05" thumb-label hide-details
             color="accent" @end="save"
           />
+          <div class="text-caption fc-muted mt-1">
+            Tagger predictions below this confidence aren't stored — raising it
+            keeps the image library lean. Suggestions can't be shown below the
+            floor; lower-confidence tags you actually want still surface through
+            the learned centroid path.
+          </div>
         </v-col>
       </v-row>
     </v-card-text>
@@ -24,17 +43,25 @@ import { useMLStore } from '../../stores/ml.js'
 const store = useMLStore()
 // 'artist' (FC-2d-vii-c) and 'copyright' (2026-06-01) retired as
 // suggestion categories; their threshold rows are gone.
+// floorMin: the per-category suggestion thresholds can't drop below the
+// tagger store floor (nothing below the floor is stored to surface).
 const fields = [
-  { key: 'suggestion_threshold_character', label: 'Character' },
-  { key: 'suggestion_threshold_general', label: 'General' },
+  { key: 'suggestion_threshold_character', label: 'Character', floorMin: true },
+  { key: 'suggestion_threshold_general', label: 'General', floorMin: true },
   { key: 'centroid_similarity_threshold', label: 'Centroid similarity' }
 ]
 const local = reactive({})
 watch(() => store.settings, (s) => { if (s) Object.assign(local, s) }, { immediate: true })
 
 async function save() {
+  // Mirror the server invariant: keep the category thresholds at or above the
+  // store floor so a raised floor doesn't leave a threshold stranded below it.
+  const floor = local.tagger_store_floor
+  local.suggestion_threshold_character = Math.max(local.suggestion_threshold_character, floor)
+  local.suggestion_threshold_general = Math.max(local.suggestion_threshold_general, floor)
   const patch = {}
   for (const f of fields) patch[f.key] = local[f.key]
+  patch.tagger_store_floor = local.tagger_store_floor
   try { await store.patchSettings(patch) }
   catch (e) { toast({ text: e.message, type: 'error' }) }
 }