feat(explore): reach dial to escape dense clusters + anti-revisit (#1476)

The Explore walk got stuck in dense signatures — neighbours all too similar, so
forward-arrow couldn't escape and Random was the only exit. Root cause: MMR only
diversifies WITHIN the nearest ~400 pool; in a dense cluster that whole pool is
near-identical, so there's no escape route in it.

- gallery_service.similar(reach=0.0, exclude_ids=None): reach>0 widens the pool
  (cap 400→1000) and _reach_sample strides across an outward-growing distance span
  so the set handed to MMR spans near→mid-far (guaranteed escape routes), not just
  the tight cluster. exclude_ids drops already-walked images. Gallery 'more like
  this' (reach=0) is unchanged.
- api/gallery similar: parse reach + exclude_ids.
- explore store: default reach 0.4 (auto-diversifies without touching the dial),
  pass the breadcrumb as exclude_ids, setReach action.
- ExploreView: a Near↔Far reach slider in the trail.
- tests: _reach_sample math (deeper ranks with higher reach, near kept); similar
  exclude_ids drops walked + reach path runs clean.

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

frontend/src/views/ExploreView.vue

@@ -31,6 +31,20 @@
           <img :src="c.thumbnail_url" alt="" loading="lazy" />
         </button>
         <div class="fc-ex__trail-actions">
+          <!-- Reach (#1476): how far each step reaches past the anchor's cluster.
+               Raise it to break out of a dense signature without hitting Random. -->
+          <div
+            class="fc-ex__reach"
+            title="How far each step reaches — raise it to escape a dense cluster without going fully random"
+          >
+            <v-icon size="16" color="accent">mdi-map-marker-distance</v-icon>
+            <v-slider
+              :model-value="store.reach" @end="store.setReach"
+              :min="0" :max="1" :step="0.2" hide-details density="compact"
+              color="accent" class="fc-ex__reach-slider"
+            />
+            <span class="fc-muted fc-ex__reach-label">{{ reachLabel }}</span>
+          </div>
           <!-- Active retrain right where you tag: fold the +/- you just gave
                into the heads without a trip to Settings (the nightly beat is the
                passive cadence). -->
@@ -153,6 +167,12 @@ const modal = useModalStore()
 
 const anchorId = computed(() => route.params.imageId || null)
 const isVideo = computed(() => !!store.anchor?.mime?.startsWith('video/'))
+const reachLabel = computed(() => {
+  const r = store.reach
+  if (r <= 0.15) return 'Near'
+  if (r <= 0.55) return 'Varied'
+  return 'Far'
+})
 
 // #1206: hovering a suggestion in the rail highlights the crop it came from on
 // the anchor image (same provide/inject as the modal viewer).
@@ -296,6 +316,14 @@ onUnmounted(() => {
 .fc-ex__trail-actions {
   margin-left: auto; display: flex; align-items: center; gap: 4px; flex: 0 0 auto;
 }
+.fc-ex__reach {
+  display: flex; align-items: center; gap: 6px;
+  margin-right: 8px;
+}
+.fc-ex__reach-slider { width: 96px; }
+.fc-ex__reach-label {
+  font-size: 12px; min-width: 44px; text-align: left;
+}
 
 /* The three panes fill the remaining height; each scrolls on its own.
    grid-template-rows: minmax(0, 1fr) BOUNDS the single row to the container