fix: variant artwork was dropped as a near-duplicate even at threshold 0 (4223)

The operator reported a 15-image variant pack landing as 3 records, then
reported variants STILL being dropped with phash_threshold at 0 — the floor
of the dial. No setting could have fixed it: at hash_size=8 a pHash is 64
bits of coarse light/dark layout, so two variants sharing a composition
produce the SAME bits. Distance 0 meant "identical hash", not "identical
image", and the dial was simultaneously too coarse to keep variants and too
tight to catch a re-encoded rescale.

The hash no longer decides a merge on its own. find_similar now runs three
gates, cheapest first: the threshold proposes candidates, aspect ratio
(ASPECT_TOL, matching the tier-1 video path) rejects crops and re-canvases,
and a pixel-level confirm on the two files accepts. Every gate fails closed —
unknown dimensions, an unreadable candidate, a hash of the wrong width all
mean "not a duplicate", because too strict keeps a redundant copy the
operator can see while too loose deletes artwork only a source re-walk
returns.

- utils/phash.py: HASH_SIZE 8 -> 16 (256-bit, what ImageRepo always used);
  aspect_matches, fingerprint/fingerprint_path/fingerprints_match (PIL-only,
  mean drift + changed-pixel fraction), find_similar gains `confirm`.
- importer: _pixel_confirmer supplies gate 3 on both dedup sites, lazily and
  cached, so a non-matching import costs no extra I/O.
- 0098: widens image_record.phash to 64 chars and NULLs every value — a
  stored 64-bit hash cannot be compared to a 256-bit one, and backfill_phash
  is NULL-only, keyset-paginated and now on the daily beat, so the library
  re-hashes itself. Dedup degrades to sha256 until it finishes.
- phash_threshold counts bits and the denominator went 64 -> 256, so the
  setting is reset to the new default of 24 (there is no honest carry-over)
  and the slider is rescaled to 0-64.
- gallery_service dup_threshold 8 -> 32: the same fraction of the hash, so
  the Explore rail keeps the variance the operator tuned in on 2026-07-01.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/gallery_service.py

@@ -322,7 +322,7 @@ def _gallery_images(rows, artists: dict[int, dict]) -> list[GalleryImage]:
     ]
 
 
-def _diversify_similar(src, rows, limit, *, dup_threshold=8, lam=0.40):
+def _diversify_similar(src, rows, limit, *, dup_threshold=32, lam=0.40):
     """Trim a nearest-cosine candidate pool down to `limit` diverse picks.
 
     1. pHash collapse: drop any candidate whose perceptual hash is within
@@ -338,6 +338,11 @@ def _diversify_similar(src, rows, limit, *, dup_threshold=8, lam=0.40):
     2026-07-01 — dropped 0.55→0.40, dup 6→8, paired with a wider pool in
     `similar()`).
 
+    `dup_threshold` counts Hamming bits, so it moved 8→32 when the pHash went
+    from 64 to 256 bits (#4223, migration 0098) — the same fraction of the
+    hash, i.e. the tuning the operator chose, unchanged. This collapse is
+    DISPLAY-only: it hides a near-dup from one rail, it never drops a record.
+
     Falls back to nearest-order (`rows[:limit]`) on any failure or a small pool.
     """
     if len(rows) <= 1: