fix(explore): diversify "more like this" so it stops getting stuck (#1188)

Pure nearest-cosine piled near-identical images into the neighbour grid — a
reposted banner filled all 24 slots, and once you wandered into a B&W /
comic-panel cluster every neighbour was more of the same with no way back to
colour without the Random button (operator-reported, with screenshot).

similar() now over-fetches a wide candidate pool (5x the requested limit, cap
200), then diversifies down to `limit`:
- pHash near-duplicate collapse: drop candidates within 6 Hamming bits of the
  anchor or an already-kept candidate, so a repost (and the anchor's own clones)
  appears at most once.
- MMR re-rank: greedily pick for closeness-to-anchor minus similarity-to-already
  -picked (lambda 0.55), so the result SPANS clusters instead of returning 40
  variations of one image. Falls back to nearest-order on any failure / small
  pool, so existing nearest-first behaviour is unchanged when there's nothing to
  diversify.

Frontend forwardTarget drops the now-redundant skip-nearest-third hack (the list
is already diversified server-side) — plain random-over-unvisited gives the
variance now.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/stores/explore.js

@@ -93,13 +93,11 @@ export const useExploreStore = defineStore('explore', () => {
     // a crumb (which snaps the cursor back into the trail — the "loops back"
     // report). Fall back to the full set only if every neighbour's been seen.
     const seen = new Set(breadcrumb.value.map((c) => c.id))
-    let pool = neighbors.value.filter((n) => !seen.has(n.id))
-    if (!pool.length) pool = neighbors.value
-    // neighbors come similarity-sorted (nearest first). Skip the closest slice —
-    // those near-duplicates are exactly what you get stuck cycling through — and
-    // pick from the more-varied remainder, for real variance in the walk.
-    const skip = pool.length >= 6 ? Math.floor(pool.length / 3) : 0
-    const cands = pool.slice(skip)
+    const pool = neighbors.value.filter((n) => !seen.has(n.id))
+    const cands = pool.length ? pool : neighbors.value
+    // The list is already pHash-deduped + MMR-diversified server-side (it spans
+    // clusters, not 40 near-dupes), so a plain random pick gives real variance —
+    // no need to skip the nearest slice the way the raw nearest-list required.
     return cands[Math.floor(Math.random() * cands.length)].id
   }