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>

backend/app/api/gallery.py

@@ -148,6 +148,17 @@ async def similar():
     # Explore passes exclude_wip=1 to also drop work-in-progress from the
     # rabbit-hole; the gallery's own "similar" button omits it (keeps wip, #1274).
     exclude_wip = request.args.get("exclude_wip") in ("1", "true", "True")
+    # Explore reach (#1476): 0 = nearest (gallery default), →1 reaches into farther
+    # distance bands so the walk can escape a dense cluster. exclude_ids = the
+    # breadcrumb, so already-walked images aren't re-served as neighbours.
+    try:
+        reach = max(0.0, min(1.0, float(request.args.get("reach", "0"))))
+    except ValueError:
+        reach = 0.0
+    exclude_ids = [
+        int(x) for x in request.args.get("exclude_ids", "").split(",")
+        if x.strip().isdigit()
+    ] or None
     # post_id is the exclusive post-detail view — not a similarity scope.
     # include_hidden is a gallery-browse flag; similar() has its OWN presentation
     # exclusion (a similarity-quality concern, #1274), so drop it here (#141).
@@ -158,7 +169,8 @@ async def similar():
         svc = GalleryService(session)
         try:
             images = await svc.similar(
-                image_id=similar_to, limit=limit, exclude_wip=exclude_wip, **scope)
+                image_id=similar_to, limit=limit, exclude_wip=exclude_wip,
+                reach=reach, exclude_ids=exclude_ids, **scope)
         except ValueError as exc:
             return jsonify({"error": str(exc)}), 400
     if images is None: