fix: ruff I001 — canonical_subdir sorts before derive_subdir

Rule 102's import-order trap, hit by inserting into the block rather than
re-sorting it.

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

backend/app/services/importer.py

@@ -33,9 +33,9 @@ from ..models import (
 )
 from ..utils import safe_probe
 from ..utils.paths import (
+    canonical_subdir,
     derive_subdir,
     derive_top_level_artist,
-    canonical_subdir,
     filehash_from_url,
     hash_suffixed_name,
     safe_ext,