fix: the single-color filter no longer takes line art for a blank image (#4483)

Five of Todding's Discord doodles (pencil lines on white, 3000px) were skipped
on import as "single color", so their posts showed text and no image. The
predicate sampled a 64px BILINEAR thumbnail, which blends thin strokes into
the paper, and 0.95 "one color" is below how white a doodle is.

- Sample 256x256 by NEAREST, so each sample is a real pixel.
- Default threshold 0.995: blank means essentially blank. Migration 0116 moves
  a stored 0.95 (the old default) with it; the settings slider now spans
  0.9-1 in 0.005 steps so the value is reachable.
- The Cleanup audit shares the predicate, so it stops flagging sketches too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

alembic/versions/0116_single_color_threshold.py

@@ -0,0 +1,34 @@
+"""The single-color filter's default becomes near-total: 0.95 → 0.995.
+
+#4483. At 0.95 the filter rejected line art: a pencil doodle on white is
+mostly white, and five of Todding's Discord doodles were skipped on import as
+"single color", leaving their posts with text and no image. The predicate now
+samples real pixels instead of a blurred thumbnail, and the default only
+calls an image blank when it essentially is one.
+
+A stored 0.95 is the old default, so it moves with it. Any other value was
+chosen by hand and is left alone.
+
+Revision ID: 0116
+Revises: 0115
+Create Date: 2026-09-28
+
+"""
+from alembic import op
+
+revision = "0116"
+down_revision = "0115"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("import_settings", "single_color_threshold", server_default="0.995")
+    op.execute(
+        "UPDATE import_settings SET single_color_threshold = 0.995 "
+        "WHERE single_color_threshold = 0.95"
+    )
+
+
+def downgrade() -> None:
+    op.alter_column("import_settings", "single_color_threshold", server_default="0.95")