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_paths.py

@@ -1,6 +1,7 @@
 from pathlib import Path
 
 from backend.app.utils.paths import (
+    canonical_subdir,
     derive_subdir,
     derive_top_level_artist,
     filehash_from_url,
@@ -59,3 +60,38 @@ def test_derive_top_level_artist_nested():
 
 def test_derive_top_level_artist_root():
     assert derive_top_level_artist(Path("/import/x.png"), IMPORT) is None
+
+
+# --- canonical_subdir (milestone #421) --------------------------------------
+
+
+def test_canonical_subdir_replaces_the_top_segment():
+    assert canonical_subdir("Conto/patreon", "conto") == "conto/patreon"
+    assert canonical_subdir("Conto", "conto") == "conto"
+
+
+def test_canonical_subdir_keeps_everything_below_the_artist():
+    """Only the artist segment is authoritative — post folders below it are
+    the downloader's business and must survive untouched."""
+    assert canonical_subdir(
+        "Big Bang/patreon/2026-01-02_123_A Post", "big-bang"
+    ) == "big-bang/patreon/2026-01-02_123_A Post"
+
+
+def test_canonical_subdir_passes_through_without_an_artist():
+    """No resolved artist means nothing authoritative to canonicalise against,
+    so the import folder's own name stands."""
+    assert canonical_subdir("Conto/patreon", None) == "Conto/patreon"
+    assert canonical_subdir("Conto", "") == "Conto"
+
+
+def test_canonical_subdir_leaves_the_images_root_alone():
+    """An empty subdir is a file landing at the root — there is no artist
+    folder to correct, and what becomes of those is task #4247's call."""
+    assert canonical_subdir("", "conto") == ""
+    assert canonical_subdir("", None) == ""
+
+
+def test_canonical_subdir_is_idempotent():
+    once = canonical_subdir("Conto/patreon", "conto")
+    assert canonical_subdir(once, "conto") == once