fix: library paths follow the artist's slug, not the import folder's name (4244)

Step 1 of milestone #421. The images tree has 57 directory families for what
the database says are single artists — `Conto`/`conto`, `InCaseArt`/`incaseart`,
`StickySpoodge`/`Stickyspoodge`/`stickyspoodge`, and so on down to a four-way
split for Pocket Ace Games.

There was never a duplicate Artist row. `/api/artists/names` returns exactly
one per artist. The files simply get written to two places for one row:
`_copy_to_library` built its destination from `derive_subdir`, which mirrors
the IMPORT tree's folder name verbatim, while the download path leaves files
where the ingester wrote them — under the slug. Two writers, two conventions,
one artist.

This is the half that stops it re-growing, and it has to land before anything
moves existing files: consolidate first and the next filesystem import out of
a capitalised folder re-creates the directory that was just emptied.

`canonical_subdir` replaces the top-level segment with the artist's slug and
leaves everything below it alone — the post hierarchy is the downloader's
business. Two deliberate pass-throughs: no resolved artist (nothing
authoritative to canonicalise against) and an empty subdir (a file at the
images root, whose fate is task #4247, not a side effect of this helper).

`_supersede` resolves the KEPT row's artist for the same reason — a supersede
rewrites `existing.path`, so writing it anywhere else would move a row back
out of the tree being consolidated. ImageRecord carries `artist_id` with no
relationship attribute, so that is a session lookup rather than an attribute.

test_import_one_happy_path pinned the old `Alice/` destination and now pins
`alice/` (rule 90).

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

tests/test_importer.py

@@ -58,10 +58,36 @@ def test_import_one_happy_path(importer, import_layout):
     result = importer.import_one(src)
     assert result.status == "imported"
     record = importer.session.get(ImageRecord, result.image_id)
-    assert record.path.startswith(str(images_root / "Alice"))
+    # Under the artist's SLUG, not the import folder's "Alice" (milestone
+    # #421) — the import tree's capitalisation is an accident of how the
+    # operator named a folder, and honouring it gave one artist two homes.
+    assert record.path.startswith(str(images_root / "alice"))
     assert record.path.endswith(".jpg")
 
 
+def test_library_path_uses_the_artist_slug_not_the_folder_name(
+    importer, import_layout,
+):
+    """The regression that grew 57 duplicate artist directories: an import
+    from `/import/Conto/...` wrote `<images>/Conto/...` while the downloader
+    wrote `<images>/conto/...` for the same Artist row."""
+    import_root, images_root = import_layout
+    src = import_root / "StickySpoodge" / "patreon" / "a.jpg"
+    _make_jpeg(src)
+    result = importer.import_one(src)
+
+    record = importer.session.get(ImageRecord, result.image_id)
+    artist = importer.session.execute(
+        select(Artist).where(Artist.slug == "stickyspoodge")
+    ).scalar_one()
+    assert artist.name == "StickySpoodge"
+    # Artist segment canonicalised; the post hierarchy below it untouched.
+    assert record.path.startswith(
+        str(images_root / "stickyspoodge" / "patreon")
+    )
+    assert not Path(record.path).is_relative_to(images_root / "StickySpoodge")
+
+
 def test_folder_creates_artist_and_links_artist_id(importer, import_layout):
     # FC-2d-vii-c: folder import creates the Artist and sets the canonical
     # image_record.artist_id — no artist-kind Tag (that path was retired;