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/models/image_record.py

@@ -64,7 +64,11 @@ class ImageRecord(Base):
     # that 0001 also built was an exact duplicate of it — dropped in 0089
     # (#3301). Lookups by sha256 use the constraint's index.
     sha256: Mapped[str] = mapped_column(String(64), nullable=False)
-    phash: Mapped[str | None] = mapped_column(String(32), nullable=True, index=True)
+    # 64 hex chars = the 256-bit hash utils.phash emits at hash_size=16. Was
+    # String(32) (64-bit) until migration 0098; the narrow column was the
+    # reason for the undersized hash, and the undersized hash was collapsing
+    # variant artwork into one record (#4223).
+    phash: Mapped[str | None] = mapped_column(String(64), nullable=True, index=True)
     size_bytes: Mapped[int] = mapped_column(BigInteger, nullable=False)
     mime: Mapped[str] = mapped_column(String(64), nullable=False)
     width: Mapped[int | None] = mapped_column(Integer, nullable=True)

backend/app/models/import_settings.py

@@ -42,7 +42,12 @@ class ImportSettings(Base):
     single_color_threshold: Mapped[float] = mapped_column(Float, nullable=False, default=0.95, server_default="0.95")
     single_color_tolerance: Mapped[int] = mapped_column(Integer, nullable=False, default=30, server_default="30")
 
-    phash_threshold: Mapped[int] = mapped_column(Integer, nullable=False, default=10, server_default="10")
+    # Hamming distance over a 256-bit pHash (utils.phash, hash_size=16). The
+    # unit CHANGED in migration 0098 — it used to be bits out of 64 — so the
+    # old default of 10 is not this scale's 10, and 0098 resets every row.
+    # This is now the cheap PRE-FILTER: the aspect + pixel gates decide, which
+    # is what lets it be generous enough to catch a re-encoded rescale.
+    phash_threshold: Mapped[int] = mapped_column(Integer, nullable=False, default=24, server_default="24")
 
     # FC-3c downloader knobs
     download_rate_limit_seconds: Mapped[float] = mapped_column(