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>

tests/test_gallery_similar.py

@@ -170,6 +170,23 @@ async def test_similar_respects_limit(db):
     assert len(res) == 2
 
 
+@pytest.mark.asyncio
+async def test_similar_exclude_ids_drops_walked(db):
+    """Explore passes its breadcrumb as exclude_ids so already-walked images
+    aren't re-served as neighbours (#1476). reach>0 runs cleanly too (small pool
+    → the sampler passes through)."""
+    src = await _img(db, 1, _vec(1, 0))
+    walked = await _img(db, 2, _vec(1, 0.05))
+    fresh = await _img(db, 3, _vec(1, 0.3))
+    svc = GalleryService(db)
+    res = await svc.similar(src.id, limit=10, exclude_ids=[walked.id])
+    ids = {i.id for i in res}
+    assert walked.id not in ids
+    assert fresh.id in ids
+    res_reach = await svc.similar(src.id, limit=10, reach=1.0)
+    assert fresh.id in {i.id for i in res_reach}
+
+
 # --- API ---
 
 @pytest.mark.asyncio