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

backend/app/utils/paths.py

@@ -60,6 +60,37 @@ def derive_subdir(source_path: Path, import_root: Path) -> str:
     return str(rel) if str(rel) != "." else ""
 
 
+def canonical_subdir(subdir: str, artist_slug: str | None) -> str:
+    """`subdir` with its TOP-LEVEL segment replaced by the artist's slug.
+
+        canonical_subdir("Conto/patreon", "conto")  -> "conto/patreon"
+        canonical_subdir("Conto", "conto")          -> "conto"
+        canonical_subdir("Conto/patreon", None)     -> "Conto/patreon"
+        canonical_subdir("", "conto")               -> ""
+
+    `derive_subdir` mirrors the IMPORT tree's folder names verbatim, so a
+    filesystem import out of `/import/Conto/...` used to write
+    `<images_root>/Conto/...` while the download path wrote `<root>/conto/...`
+    for the very same Artist row. One artist, two directories, forever — 57
+    such families had accumulated by 2026-09-21, and the database never had
+    duplicate artists at all (milestone #421).
+
+    The slug is the canonical name because it is the Artist row's own
+    identifier: it is what `/api/artists` reports, what the ingesters already
+    write, and the one spelling that cannot vary with how a folder happened to
+    be capitalised on the way in.
+
+    Two deliberate pass-throughs. NO artist resolved means there is nothing
+    authoritative to canonicalise against, and an EMPTY subdir is a file
+    landing at the images root — those have no artist folder to correct, and
+    what becomes of them is its own decision (task #4247), not a side effect
+    of this helper.
+    """
+    if not artist_slug or not subdir:
+        return subdir
+    return str(Path(artist_slug, *Path(subdir).parts[1:]))
+
+
 def hash_suffixed_name(stem: str, sha256_hex: str, ext: str) -> str:
     """Builds 'stem__<first10ofhash><ext>'.